It's not just here, a lot of gaming forums I visit are on the decline. I'm on IGN as well and the Nintendo discussion section there is absolutely glacial compared to, say, 4-5 years ago.
Resetera on the other hand, is still very active, and their moderation is extremely strict. Not sure what their secret is, how they manage to thrive under such conditions.
Simple - they took the most popular forum and expanded. Others are in decline because the landscape has changed and they need to evolve with it.