I'm trying to think what's changedAmong other possibilities:
SO suffers from hostile moderation and a generally unwelcoming culture, perhaps even worse than Wikipedia. This has a profound chilling effect on positive, substantial contributions, particularly from new contributors.
SO had a great strategy initially with relying on search engines to index everything, but it never seems to have solved the recency/relevance problem. In that respect, it has become its own worst enemy, with old answers about obsolete versions and practices often ranking highly in search results.
The first of those problems then exacerbates the second, because the same cultural issues get in the way of both updating answers to old questions and asking new questions that might be superficially similar to ones that already exist but actually need a different answer.
Meanwhile, in many areas of programming, documentation from other sources has become both better in quality and more readily located thanks to other well-known sites and high search engine rankings. Relatively speaking, SO simply isn't as useful if there is already primary documentation that answers questions correctly and comprehensively.
And finally, you personally may have grown as a developer over time, becoming both more capable of solving problems for yourself and more familiar with whatever tools you use regularly, so you might not need external help so often.
FWIW, I'm also in the "rarely visit SO any more" camp. I think I have a kind of banner blindness for SO hits on search results pages now, perhaps because I'm assuming that following a SO link is unlikely to provide a useful answer so I almost always check other plausible sources first. On those occasions when I do get as far as visiting SO, I'm usually reminded of why I tend to work this way now.