I mostly blame the near-complete abandonment of the product. It was quite clear even early on that it did not work very well for time-sensitive knowledge, and that having any of that cluttered the search results badly, but nothing at all was ever done to address that, despite nearly all of Stack Overflow fitting in that category.

IMO the uselessness of the answers just became more and more apparent, so fewer people contributed, so they rotted further. Good riddance, I say - it had a brief couple years where it was excellent because all information was new enough to be useful, and then it started dying like all stagnant things do.

Stack Exchanges that do not have such a bias for time-sensitive information are still going relatively well AFAICT, e.g. math exchange. The model works fine for some things, just not general tech assistance.