There is a dreamer in everyone

Edit, because I realised that I never explicitly stated this: I personally believe that Elain’s lack of interest in her bond with Lucien, and Lucien himself is, in itself, a rejection. She should not need to do more than not give consent to any sort of relationship with Lucien to "reject" him, or their bond. The following discussion is based upon their "in-world" situation, as faeries with a mating bond that could potentially require more than just not pursuing a relationship to consider one rejected.

We haven’t yet had Elain’s book 

The first, easiest and most obvious answer is that SJM wants to tell this story from Elain’s POV - or, potentially, Lucien. We are yet to see a book that features Elain’s POV, and we only got a few pages of Lucien’s POV, courtesy of Feyre and her daemati powers, towards the beginning of ACOWAR (chapter 24). Since then, nothing. 

Besides the fact that Elain was in no shape to make a decision about her bond with Lucien at that time - in addition to recovering from the trauma of being kidnapped, forcibly turned into a faerie, and being abandoned by the man she loved, who supposedly loved her, for events over which she had no control, she spent a lot of her time lost in visions that we later found out were prophetic - I would expect anyone to take a step back and decide for themselves what such a significant bond would mean, both to and for them. 

It is the smart and responsible thing to do, otherwise she, or even Lucien, could end up like Rhys' mother, who resented his father, even though they were mates. If that means two years of an immortal lifespan spent contemplating whether she wants to let him get to know her, then so be it. 

I sighed, absently rubbing my still-flat stomach. "Let's focus on helping one sister before we start on the other." "Agreed," Rhys drawled. - ACOSF, Feyre’s bonus chapter 

And if none of the above makes sense to you, it simply hasn't happened yet because it would be such a momentous occasion - SJM hasn’t written a mating bond rejection before - that I cannot fathom why we would expect to see it from anyone other than the two people involved. My money is on Elain learning that she can Unmake the bond in ACOTAR 5, and doing so from her POV, then we will get Lucien’s POV either as a bonus chapter in ACOTAR 5, or as a flashback in ACOTAR 6, if he shares that with Vassa. 

Elain knows she can reject the bond, but she is afraid of hurting Lucien 

Women today are still taught, often subconsciously, to centre men - and how they might respond to a situation - from a young age; for example, dress codes at schools that unfairly target the female students, so that they're not a distraction. As we get older, we also tend to automatically consider a man’s potential response in how we would refuse an advance, in terms of both our personal safety and not wanting to cause emotional harm - I have done this more than once, and so have many of my friends… and we live in "safe" areas. Please understand that I am not suggesting that Lucien (or the majority of men) would be violent towards Elain (or any woman), or see her as property; he has fairly consistently respected her space for almost two years now. 

However, Elain is frequently described as sweet and kind - she was the first sister to openly state and apologise for her part in Feyre’s neglect growing up, without prompting or design; she was raised in a society that is more patriarchal than our own current western world, and by a mother who conditioned her to be a pawn on the marriage market, to boot. I would put money on the Archeron sisters all having some sort of internalised response similar to women that live in the western world today - and I am using the western world, because that is where the author lives, and what I personally know - which would mean that Elain, who doesn't like causing people pain, could feasibly know that the bond can be rejected, but is stalling because she doesn't want to hurt Lucien, either physically or emotionally, and so doesn’t know how to proceed. 

Now, obviously an ignored bond would be causing some base-level of pain, at least emotionally, but Elain is between a rock and a hard place, and I could see her not knowing how to proceed. She probably remembers the devastation that Graysen made her feel, and is reluctant to pass that on, even though we know she'd never be cruel about it. 

I glowered at Rhys. "You think Elain’s boring?" "I think she's kind, and I'll take kindness over nastiness any day. But I also think we haven't yet seen all she has to offer." - ACOSF, Feyre’s bonus chapter 

Elain knows she can reject the bond, but she is worried about her safety  

Following on from the second point, that Elain doesn’t want to reject the bond because it could hurt Lucien, either emotionally or physically, I posit a twist on the same idea; Elain is concerned about her safety, due to the backwards and sexist nature of the Autumn Court. 

“Not free rein—not in my lands. It has been illegal in our territory for a long, long time for males to do that. Even before I was born. Other courts, no. On the continent, there are territories that believe the females literally belong to their mate. But not here. Elain would have our full protection if she rejects the bond. But it will still be a bond, however weakened, that will trail her for the rest of her existence.” - ACOWAR, chapter 24 

I clenched my teeth. “If you were her lover, why didn’t you stop it?” The wrong thing to say. Utterly wrong, by the dark fury that rippled across Helion’s face. “Beron is a High Lord, and she is his wife, mother of his brood. She chose to stay. Chose. And with the protocols and rules, Lady, you will find that most situations like the one you were in do not end well for those who interfere.” - ACOWAR, chapter 47 

Once again, I highly doubt this is due to Lucien himself, but his father, Beron, is terrible - look at what he did to Lucien’s first love, Jesminda - and he knows that Elain is Lucien’s mate. We know that Rhys told Feyre that Elain would be protected, and that no bond would be forced on her, which could be hinting that it is something Beron may consider, either out of shame that his (as far as he knows) son could be rejected by his mate, or as a power play over the Night Court/the rest of Prythian. 

Aside from the fact that Elain is Lucien’s mate, she is also a Seer in her own right; she is Cauldron-Made, and likely has additional powers, if Amren is to be believed. Elain would be sought out by many Courts and Kingdoms if they knew of her, and if Beron thought he could stake a claim to such an asset, I am sure he would take her with little thought to her own wishes, or even Lucien’s. If Beron is working for Koschei, and either Koschei or he got wind of Elain’s abilities, then Elain could be in big trouble. 

And do not forget that Nesta herself—and Elain, with whatever powers she has—is here. Feyre is here. All three sisters blessed by fate and gifted with powers to match your own. - ACOSF, chapter 42

Elain knows she can reject the bond, but she is worried about political backlash 

Similar to the above situation, in which Elain fears for her own safety, I can see her also stalling in rejecting her bond with Lucien because she is worried about the safety of others; she has always loved her sisters, and has come to love the Inner Circle, and the Night Court in general and, despite her reservations towards Lucien, I highly doubt she wants to see him harmed. She certainly wouldn't want Azriel to get hurt, especially on her account. 

Rhys growled, "Allow me to make one thing very clear. You are to stay away from her." "You can't order me to do that." "Oh, I can, and I will. If Lucien finds out you're pursuing her, he has every right to defend their bond as he sees fit. Including invoking the Blood Duel." "That's an Autumn Court tradition." The battle to the death was so brutal that it was only enacted in rare cases. Despite being an outsider, Azriel had wanted to invoke it when he'd found Mor all those years ago. Had been ready to challenge both Beron and Eris to Blood Duels and kill them both. Only Mor's right to claim their heads in vengeance had kept him from doing so. "Lucien, as Beron's son, has the right to demand it of you." "I'll defeat him with little effort." Pure arrogance laced every word, but it was true. "I know." Rhys's eyes flickered. "And your doing so will rip apart any fragile peace and alliances we have, not only with the Autumn Court, but also with the Spring Court and Jurian and Vassa." Rhys bared his teeth. "So you will leave Elain alone. If you need to fuck someone, go to a pleasure hall and pay for it, but stay away from her." Azriel snarled softly. - ACOSF, Azriel’s bonus chapter 

As Rhys said in Azriel’s bonus chapter from ACOSF, if a relationship between Elain and Azriel was ever discovered, Lucien - not Azriel - could call a Blood Duel, which is still a valid custom in the Autumn Court. This would put both Azriel and Lucien at risk of injury or death, as well as disrupting the tentative alliance between (most of) the Courts of Prythian.

I personally don't think that Lucien would call Azriel out for a Blood Duel unprompted, it would most likely be due to Beron's machinations, or Beron himself. Furthermore, Lucien is better-known throughout Prythian than anyone from the Night Court; again, I doubt he would do this, but with his personal and political connections, he could do some real damage to the Night Court's reputation. 

Elain is observant - the Cauldron saw fit to make her a Seer, and her two best friends are spies, after all - so I would be very surprised if she was unaware of what was going on around her. 

Elain almost certainly knows the importance of political accord across Prythian, especially given the upcoming conflict with Koschei and/or the Autumn Court under Beron’s rule. Given she likely put herself on the marriage market, as a human, to both provide a safe place for her and Nesta, and contribute to the war effort against the faeries who were set on invading the Human Realm, this savviness tracks. Elain deciding to act in the way that is perceived as best for her Court/family would not be out of character for her.

Elain knows she can reject the bond, but has Seen a reason not to do so… yet 

Elain is a Seer - who likely has other powers - and, though we have heard her speak a few of her prophecies out loud, I am almost certain that she has Seen more than she has let on, at least to us. 

Mor let out a snort that made the Illyrians stiffen. But she shifted, revealing Elain behind her. Elain was just blinking, wide-eyed, at the camp. The army. Devlon let out a grunt at the sight of her. But Elain wrapped her own blue cloak around herself, averting her eyes from all of those towering, muscled warriors, the army camp bustling toward the horizon … She was a rose bloom in a mud field. Filled with galloping horses. - ACOWAR, chapter 51 

I have a feeling that any time Elain blinks, or tilts her head, it could be signifying her having had a vision - this is discussed further in this fantastic post. The scene above, in which she blinks at Windhaven, and averts her eyes from the warriors, is likely foreshadowing what the Cauldron did the the Illyrian army, during the battle with Hybern, or potentially something that has not yet come to pass. What has she Seen, or what does she know/suspect, that could be behind her choice to ignore her bond with Lucien? Given we know she doesn’t like causing anyone pain, it must be something huge. 

Does the Night Court need Lucien himself in some manner going forward?

Is it vital that the Night Court maintains its ties to Lucien, and therefore Queen Vassa, the firebird captive of Koschei (and Lucien’s suspected love interest)?

Will the bond between Lucien and Elain play a significant role in the near future, by one of them sensing the other in danger? Many suspect that Azriel felt something when Elain was kidnapped by the Cauldron, and then magically contained by the King of Hybern… but did Lucien also feel the bond go quiet and, if so, does it work both ways?

If Vassa and Lucien’s story is going to be based on Swan Lake, will Lucien’s declaration of love - to break Vassa’s curse - come in the form of a mate bond that has to be freshly Made? 

These are just a few ideas; there are many plot-driven points that could explain why Elain has not rejected her bond with Lucien… yet.  

Elain knows she does not have to accept the bond, but doesn’t know that it has to be officially declined

None of us actually knows what is involved with rejecting a mating bond, just that a mate bond can be declined, but not broken - and that the fae involved, especially the male, might still feel a reduced pull. 

Elain is an astute person who has embraced her new, fae life in the Night Court, but it is very possible that, while she is aware that she does not have to accept her bond with Lucien, she is unaware that it has to be rejected in an official capacity. We, the readers, don’t even know whether this is the case. 

Despite what we have seen so far in the ACOTAR series, mate bonds are supposedly rare; I would assume that rejected mate bonds are rarer still, even though there are canon reports of accepted mate bonds going sour - for example, Rhys’ parents. I wonder how many people would actually know what is involved? The priestesses, maybe, as they are involved in mating ceremonies, but I doubt that ‘how to reject an unwanted mating bond’ is common knowledge, or regular tea time conversation.

Elain knows she can reject the bond, but is (or was) undecided 

I don't personally want to see this, because it saddens me to think that Elain would value herself so little, but I could see that years of conditioning by her mother, and society in general, would have Elain thinking that she should give Lucien a chance, even if she isn’t interested. 

Elain only stared at the steaming kettle as she set it on the stone counter. “He brought you a present.” Those doe-brown eyes turned toward me. Sharper than I’d ever seen them. “And that entitles him to my time, my affections?” “No.” I blinked. “But he is a good male.” Despite our harsh words. Despite this Band of Exiles bullshit. “He cares for you.” “He doesn’t know me.” “You don’t give him the chance to even try to do so.” Her mouth tightened, the only sign of anger in her graceful countenance. “I don’t want a mate. I don’t want a male.” She wanted a human man. - ACOFAS, chapter 18 

While we know that Elain very clearly stated her opinion in ACOFAS, that she did not owe Lucien her time, she wouldn’t be the first woman (or female, in this case) to publicly state a strong opinion against a patriarchal norm, yet continue to struggle with it in private. 

As I mentioned earlier, Elain (and Lucien) are fae, with functionally immortal life spans - what is two years to them, for Elain to take the time to both know her own mind in the context of her past conditioning to become a “lady wife,” her new life as a High Fae, and her feelings for Graysen, Azriel and Lucien, and then make her decision?

Mating bonds are for life; an immortal life! Elain is neither “stringing Lucien along,” or “cheating on Lucien with Azriel,” as some of her detractors like to say; besides the fact that SHE IS NOT PROPERTY, so owes no loyalty to Lucien or their - currently unaccepted - bond, she could simply be taking the time to know her own mind. I will not fault her for that. 

Elain doesn’t know the bond can be rejected

The last option, and the one that I, personally, am least interested in seeing, is that Elain has no idea that she can decline her bond with Lucien in any way, shape or form. We saw Rhys comforting Feyre with the knowledge that Elain would be given protection in the Night Court, but we have not seen anyone pass that information on to Elain herself. 

Does Elain even know that she doesn't have to accept Lucien? All we know that she knows is that the King of Hybern was unable to sever Feyre and Rhys' mate bond.

“Not free rein—not in my lands. It has been illegal in our territory for a long, long time for males to do that. Even before I was born. Other courts, no. On the continent, there are territories that believe the females literally belong to their mate. But not here. Elain would have our full protection if she rejects the bond. But it will still be a bond, however weakened, that will trail her for the rest of her existence.” - ACOWAR, chapter 24 

In normal circumstances, I wouldn’t see this as the obvious reason as to why Elain hasn't responded to her mating bond with Lucien, as she has shown herself to be observant, proactive and wise - but she is not in "normal circumstances." So, is it a plausible basis for her having made no move to formally reject her bond with Lucien? Yes, very much so. I will also remind you that, while Elain has so far only ignored her bond with Lucien, and only given Lucien the attention she needs to, to be polite, those actions in and of themselves are a rejection, even if they are not specifically what needs to happen in the form of a mating bond, or whatever may be between them.

We cannot expect Elain to know everything about a society into which she was thrown against her wishes; she has come such a long way in under two years, with making friends and learning at least some of their customs (see: her discussion with Feyre about winter solstice in ACOFAS), but maybe she hasn't been quite ready to tackle the idea of a mating bond that she doesn't want.

I don't personally believe that it is infantilising Elain to consider the fact that Feyre and Rhys may have withheld information from her - they're currently doing the exact same thing to Lucien, after all. If anything, Feyre and Rhys would be removing her agency by choosing not to give her the information she needs to make informed decisions about her life (if this is indeed what has happened).

***

Whatever Elain decides to do with regards to her mating bond with Lucien, I'm sure there is a reason as to why she has not yet made her decision, and that we'll learn about it in her book. If that reason turns out to be as simple as "she wasn't ready to deal with it yet," then so be it; she deserves all the time she needs to process her change, and everything that entails.

I do hope, if it turns out that she doesn't yet know about the possibility of officially rejecting the bond - if that is indeed an option - that she requests/seeks out this information herself; it would show growth on her part, which is what we'd expect to see in her book. I can see her frustration at not being able to begin a relationship with Azriel (for the aforementioned political reasons) as being part of the impetus she needs for this.

If you can think of any other reasons why Elain might be waiting to decline - or accept - the El*cien bond, please add them in the comments!