There are two main reasons for the decline of SimHQ:
(1) PWEC. It's the most popular area of SimHQ currently and because of that you lose a good set of people who would otherwise join/stay/contribute. You can't mix politics and flight sims - it doesn't work, never worked and was a mistake by guod to allow it. Internet politics where seniors are alt-right radicalized using warmed-over infowars/dailycaller headlines is fine by itself (it's a free country etc), but it's just too polarizing to think you'd attract new flight sim fans where that's the most popular new content.
This makes no sense. No one is forced to go to PWEC and post there and it's not like posts made in PWEC are transferred over to Community Hall or any other forum. So in other words, if someone hates politics and doesn't want to discuss it on SimHQ, they can very easily avoid it by simply not going to PWEC. I also find it amusing how you try to conceal your obvious political bias.
Oh, and if you go purely by post count, Community Hall is significantly busier than PWEC so you may want to do your research first before posting a claim like that.