For me, I think I left for a few reasons. All of them are some flavor of "the design is user-hostile".

The first reason is the "traps" they put in the moderation review queue process. These always felt like shit, even if you were right. And for some reason they frequently picked questions where you could reasonably pick multiple reasons to close a question... and they'd yell at you for getting it wrong! Really, this needlessly punitive experience is the heart of why SO was so awful, because everything is like that across the whole site. Some of the network sites aren't like that, but it's rare. And there were just so many Meta questions where the upvoted response was "this would be hard to do right, so instead we're going to do it the way we all know is wrong but it's easy."

The second reason is just how awful the onboarding is. The site constantly had problems with people either not posting code or posting screenshots of code. I don't know why they didn't make a beginner form, similar to what you often see on GitHub issue trackers for large projects (i.e., always include this, always include this, include this if it's relevant, etc. and it's in the textarea box when you start a question). Instead they assume that you know how to ask a question, give you a completely blank form, and then aggressively policy anything where you didn't do it right. I think the one thing to come out of that site is that people do not inherently know how to ask questions. [Which, if anybody has been responsible for responding to IT tickets, oh, boy, do they not know how to ask questions or report problems. "I'm getting errors running my report in the Finance application. I need this report soon! Help!" No mention of which report. No mention of what the report parameters were. No mention of which error message. They don't know what the error message means so, obviously, it must not be useful to anybody.]

The third reason is the site finger-pointing. "This doesn't belong here so I'm locking and closing your question," is possibly the most ridiculous thing ever. Why doesn't it move the question to the other site and then make a stub link? More awful, user-hostile experience.

But, I think the final nail in the coffin is just how aggressively awful rpg.stackexchange.com is, which seeks objective, hyperliteral answers to game questions that are, by design, subjective and intentionally meant to be played with interpretation and nuance. The result is a handful of highly active users will nit pick any answer or question they don't like. Really, I think it's the GameFAQs Forum clique problem, to a significant extent. Some people really have nothing better to do except quash the utility of a site they have a small amount of control over, and it largely ruins the whole experience for the majority of other people. And, unlike a subreddit, you can't just go make your own version on the same topic. It doesn't work like that.