I know everyone here is heckling calling it a forum, but this is a basic forum feedback loop -- the forum activity declines, so people show up to check less often and get fewer responses to their posts, so the post there less often, repeat until traffic falls near zero.

You may have like a handful of weirdos who never leave and develop their own little community in the wreckage, especially if the cost to continuing to run the forum is trivial, but it's basically a death spiral every time.