Note that most of these are just regular people with close-to-vote powers, not elected or appointed moderators with special powers.

I think one major problem is that there is an entire class of people who rarely or never ask or answer questions, or even comment, and all they do is "moderate" the site by closing questions. Literally all they do. Some of these have extremely specific and narrow views on how the site "should" be. I absolutely hate it: who the hell are you? You're not even using the core function of the site. Fuck off trying to tell me how I "ought" to be using it. I don't want to gatekeep who is or isn't "part of the community", but people gatekeeping how the site can be used without actually using the site is just absolutely toxic.

There are a number of other issues as well. I can go on for a long time. But to be honest I no longer care: the site has been taken over by nihilistic capitalists who care not one iota about any aspect of the site other than the ability to earn a buck (previously it was a commercial enterprise as well, sure, but it wasn't 100% about earning a buck and many in leadership positions genuinely cared about "doing right by our community" as well). And that is probably just as much of a reason for the decline of Stack Overflow as anything else.