I think a big problem was how new users had to unlock things like the ability to comment. Probably a lot of new users really should have added comments to previous questions to clarify things, but instead the site tells them to create a new question first to get reputation points. So they do, but what they want isn't really a unique question, just clarification on a previous question.

Once you get enough reputation to be "in," suddenly the whole site opens up and you can do everything you need to. But a new user has to get to that point, and that is daunting if they're new to programming.

I also think that SO selling their data for training AI really rubbed a lot of old timers the wrong way too. If they had not given in to that, I wonder if the decline would have been nearly as sharp. There were users active there daily, finding questions to answer and evaluating others answers. Now there really aren't.