There is another thread on 4plebs, which was also active but it also died out with time. swfchan's activity can't be called great either.

I don't think there is a single reason why people stop posting.
I think most people are just not youngsters with a lot of time anymore, and finding motivation to write up a single post can start becoming a challenge, let alone creating a whole new flash (;＾Д^)
If you check the lurker counter, people still seem to check the board at least.