ChatGPT. Both that people use it instead but also that some people try to game SO with answers from it. So the moderation of the site has probably become harder.

SEO is another factor. At least I don't get as many results on SO as I used to. That may be because of many reasons but if the popularity of a site starts to fall Google search might speed that up.

EDIT: I see now that the drop started long before ChatGPT became relevant. But it won't help SO I think. Github copilot and Google featured snippets may also be factors.