Some commenters suggest it's not the moderation. I think it is the key problem, and the alternative communities were the accumulated effect. Bad questions and tough answer competition is part of it, but moderation was more important, I think. Because in the end what kept SO relevant was that people made their own questions on up to date topics.

Up until mid-2010s you could make a seriously vague question, and it would be answered, satisfactory or not. (2018 was when I made the last such question. YMMV) After that, almost everything, that hadn't snap-on code answer, was labelled as offtopic or duplicate, and closed, no matter what. (Couple of times I got very rude moderators' comments on the tickets.)

I think this lead some communities to avoid this moderator hell and start their own forums, where you could afford civilized discussion. Discourse is actually very handy for this (Ironically, it was made by the same devs that created SO). Forums of the earlier generation, have too many bells and whistles, and outdated UI. Discourse has much less friction.

Then, as more quality material was accumulated elsewhere, newbies stopped seeing SO on top of search, and gradually language/library communities churned off one by one. (AI and other summaries, probably did contribute, but I don't think they were the primary cause.)