i reanimate your bones

Steve is the one the writers believed had room for improvement. Ergo, Steve's personal journey involves challenging his worldview and perceptions of others, which allows him to grow as a person. Meanwhile, because they wrote Jonathan as "self-aware," they didn't believe he needed the same treatment as Steve. As I noted in that analysis I linked to, the problem with what they did from a writing perspective is it didn't leave much room to give Jonathan any meaningful character development. It's why Jonathan ended up stagnating (and eventually regressed) as that lack of growth (and having Jonathan hold onto the same limiting, toxic biases for multiple seasons) made itself apparent as the show went on.

Also, controversial opinion on my end, but I suspect Jonathan may have been intended by the Duffer Brothers to be the catalyst for Nancy's character development. I base that on the outline they had for the original Montauk Pilot. The problem, however, is once they made the changes they did, which eventually lead to what we got in season 1, it's no longer Jonathan who fulfills that role for Nancy. It's Barbara.

Barbara's disappearance and death is what kickstarts Nancy's journey. Nancy is the one who spends season 1 trying to find her friend, and she's also the one who approaches Jonathan later when she sees the picture of the Demogorgon behind Barbara. Jonathan works with Nancy because they have shared goals of finding people they care for who were taken by the Demogorgon, but at the end of the day, Barbara is the main drive behind Nancy's motivations. Not Jonathan. Same goes for season 2 when Nancy wants to expose Hawkins Lab for their role in Barbara's death, and how that eventually leads into Nancy's arc of pursuing investigative journalism to not only provide closure to Barbara's parents, but also avenge those who deserve justice in their names. Again, Jonathan is along for the ride because of what happened to Will, but the show makes it pretty clear that Nancy was aching to get the truth out about Hawkins Lab for a while at that point, and would've pursued this course of action with or without Jonathan or Steve by her side.

And I know fans like to cite that godawful misogynistic argument in "The Flea and The Acrobat" where Jonathan makes insinuations about Nancy and her relationship with Steve to frame it like Jonathan knew what was best for Nancy, but this is classic projection on Jonathan's end with a self-serving agenda: Jonathan wants Nancy to be with him, and he isn't above weaponizing the knowledge of her parents' loveless marriage (which is information she shared with him in a moment of vulnerability) to make it seem like Nancy would end up exactly like her parents if she continued to be with Steve. The irony, of course, is that Nancy/Jonathan's relationship ended up becoming a lot more like Ted/Karen (and even Lonnie/Joyce to a certain extent) by the end of the show in how miserable they were together, and how both Nancy and Jonathan ended up exhibiting the same toxic traits their parents had. However, that's not something fans want to hear because it doesn't paint Jancy in a favorable light. And that's not even getting into how Jonathan stopped being supportive of Nancy when her interests didn't align with his and started to inconvenience Jonathan and his family (as we saw in The Hawkins Post arc during season 3, as well as in the tie-in novel One Way or Another).

I used to see, it felt so real.