#it's the plot

Now there's the first obvious reason for this which is misogyny, both out and in universe. In universe it all links back to Hiram as always. In his constant attempts to control and get the attention of Veronica he in fact does! Gets more and more important scenes with her and takes over the use of it. Hiram Lodge is the patriarch everyone knows and cares about, good and bad, mostly bad, so he gets to take claim of the word as Hermione sits back and hates Veronica for what Hiram's taking from the both of them.

Out of universe it's well... People falling for it. Falling for the narrative Hiram (and Hermione) spins that Veronica cares more about him. People who cannot understand her and cannot understand her mom's importance in her life. People who think Hiram is all she cares about, that she cares about Archie more than Betty because they can't equate relationships that lack a mans involvement to ones that do.

So the in universe misogyny and the audience watching that sees or falls for it are some reasons. But there's additional points to how this twists the names up.

Just like with everything Hiram calls Veronica 'mija' ties into the grooming. It simply means 'my daughter' so really it could be seen as a milder one than many other things he calls her (I mean at least he never calls Hermione 'mija' unlike some things). But there's many more connotations to it. Not everyone uses 'mija' for simply their daughter, in fact it's probably more common to use it simply referring to a younger woman. I mean you don't exactly only, if at all, call your brother bro. It can actually, and unsurprisingly given that The Horse is here, often be used in a sexist context given that, I mean you rarely expect a term for a young woman to actually be endearing. 'Mija' for Hiram and Veronica is a way to put her down while being affectionate still, it is the term of someone who is above her, has control over her, takes claim to her, yet also having been something she was probably also called while being gushed over as young. The control aspect of the name very much exists with Hermione too of course, hence her first use of it in 1x03 being telling Veronica What To Do, but it lacks the grooming, or extremity of Hiram, it's caring too, at least that first time when Hiram is not relevant. (If Hermione wants to comment on Veronica's relationship with Hiram she is famously a lot more blatant. "Daddy's little girl" she uses at least twice.)

Ronnie on the other hand is just a sweet little nickname! Probably that her mom also came up with when she was little, still uses, it comes with both a connotation or Veronica as a child and a peer. Nicknames are generally more associated with children, especially one structured like Ronnie, and given how it probably started when she was little too it ties back to it but without the sense of control that comes with 'mija'. Ronnie is a term and nickname of a caring mother, Hermione first uses it when she's genuine and worried and trying to talk to her about the dance and school like a normal mom. But with this it also becomes a bit of the name of a peer of course, partially because of it being a nickname that only(?) her friends(/boyfriends, more on this soon) use and because of how Veronica's relationship with her mother is. In season 1 we meet them in their Gilmore Girls era, single mother and her daughter she is tight knit with, Hiram is forever looming and messes with their relationship but they slot into this image and type of relationship really easily. Probably because this is how it's been the majority of Veronica's life, Hiram busy working and Hermione as the main present parent. Which changes the relationship one has with your parent, you do become more of a peer, more support, and if the relationships good, well your parents your friend. Something some would call sad and Veronica would hit them over the head with a rock for. It's her mom!

Heyheyhey I'll have you know I'm getting better about that

Nope. It refers to the 7 seconds before the end of the planet.

This feels like bait, but on the off chance it isn’t- think about it. Really think about it. Let’s say they started making the ST with only two pieces of definitely decided information: 1. Rey is Nobody (confirmed by the filmmakers) and 2. Ben is the Last Skywalker who must be reclaimed (obvious), what does that mean for the structure of the plot and character arcs? What is literally the first decision which has to be made about these, the two most plot-important and theme-defining characters?

The relationship between them.

So, like how Jenova used Tifa's memories of Cloud to construct part of his soldier persona, it also used Aerith's after that flash to augment them, which is the source of the LTD.