Many fan made community Halo forums have a much more active user base. Why do so few people post here?
A lot of people post here.
Where did you get that idea? These forums are really active.
This forum is very active. There’re over two hundred thousand forum users.
You should check out the Halo 4 forums. 
This forum is incredibly active! Sure, not a whole lot of users have incredibly high post counts, but this is one of the faster-moving forums that I’ve seen!