Of course people who hate Stydia and love Stalia don't understand development. Stalia has no development and they think it's great.
I don't always have an issue with a lack of development to characters getting together. It's kind of a case by case basis thing, though. Sometimes people don't have deep friendships first. Sometimes people don't know each other very well before they start dating. So there are times when I don't mind that there isn't some big lead up. Especially when it comes to teenagers.
However, even considering that, the beginning of Stalia is still a mess. It's not just that there was no development (which there wasn't). It was partially the way it started in Eichen House. Two teenagers, one who is possessed by an evil spirit and one who murdered her family and was living as an animal until only a few weeks prior, having sex in the basement of a psychiatric hospital. It doesn't make sense character-wise. Malia should not have been in a place where she was able to give full consent for something like that. It doesn't make any sense that she was as coherent and cognizant as she was. And it doesn't make sense for Stiles to think that was a good idea. I've seen people pointing to when he was going to just have sex with the girl at the beginning of season three in the wine cellar (I can't recall her name off the top of her head), as if that explains the complaint away. It doesn't, because it's not the same kind of situation. Yes, it would probably be in character for Stiles to jump at the chance of sex if both he and the other person were willing and in a position where they could fully consent. It does not make sense for Stiles to just jump into a sex with a girl when both are in an incredibly emotionally unstable place, and especially while he's smack dab in the middle of trying to solve a huge problem/mystery.
Not only does the Eichen House hookup make no sense, it's also incredibly unhealthy, as these two teenagers in the midst of extreme emotional duress should not be entering into romantic and sexual relationships. That's not healthy.
And then in season four, they're together. Despite the fact that neither should be in an emotionally stable enough place to commit themselves to a relationship.
Not only that, but there doesn't seem to be much reason that they would like each other. I might not mind a lack of development, as long as by the time the characters are in a relationship we can see why they like each other. But other than the sex, there aren't really any reasons displayed as to why they like each other. Malia displays behavior that Stiles has been shown not to like in others, but in this case it gets explained away because she's "learning". The only other thing we've been given is the contrived scene about the highlighter and how they both use red for "I don't know".
And even if there isn't development leading to a relationship, there needs to be development within the relationship (especially if the relationship doesn't have much development in its lead up). We haven't had any of that either. A lot of that is probably because Malia has had no development. So much of the relationship is clearly Stiles trying to teach Malia how to behave, yet there has been no development on that front, no "progress" that Stiles assures is happening. Their relationship is stagnant. Even a break up, which could have and should have allowed for development, as Stiles broke Malia's trust, led to nothing. No development. The relationship is completely stagnant. And that's the kindest word that can be used to describe it, because there's a hell of a lot of problematic material in regards to the imbalances of power and their sexual relationship.
The problem with Stalia isn't that there was a lack of development leading up to it. It's that there has been no development in any way whatsoever.