Show me the multiple castle forums that exist for fans. There may be a lot, but Bionicle is at least a little bigger.
Excuse me, the burden of proof is on you. That's how this works, you made a statement, you have to back it up.
That's... Definitely not true. There are multiple contests and friend circles surrounding Castle and it's very much a part of the community, not as massive anymore as something like Ninjago but still influential. Castle fans just tend to be very quiet and not too vocal about what they do, except for the absolute masters at MOCing who put the rest to shame.