Lots of the comments here are attributing the decline to a toxic community or overly-strict moderation, but I don't think that that is the main reason. The TeX site [0] is very friendly and has somewhat looser moderation, yet it shows the exact same decline [1].
[0]: https://tex.stackexchange.com/
[1]: https://data.stackexchange.com/tex/query/1926661#graph