I'd guess three things:
1. A number of posters have left UA for AA after the merger, so the number of long-time active posters on here has declined
2. Competition for ft has increased - the other forum and blogs seem to have drawn the interests of a sizable group of long-time posters.
3. There continues to be a very high level of negativity on this forum. There exists some subset of posters here that want to relitigate the merger and how superior pmUA was in every single thread possible.
Even though I agree in part with the posters about the effects of the merger, continually refighting the merger is tiresome, unconstructive, and doesn't do anything to help advance the goals of this forum, which is flyers helping flyers.
Just the other day on the
CLE-PDX new route thread one of those negative posters tried to turn the thread into how boring and generic the PCs are, which had nothing at all to do with the discussion among a group of CLE and PDX-based posters who were excited about the new non-stop.
I think this kind of negativity scares away forum newbies and wears down other, helpful posters.