I don't know if any more responses are needed here, but I guess I'll chime in.
First, the reason I'm not around much anymore: things have changed a lot for me. I joined these forums almost 8 years ago, so I guess it's pretty obvious that would happen. I was only 12 years old when I first came here. My interests have changed a lot since then, to the point that I barely even play video games anymore. I think part of the reason I'm still here is because it played such a major role in growing up. I didn't have a ton of friends in school, so this is really where I went to talk to people. I made some good friends over the years, and being here slowly shaped me into who I am today, at least online. In a way, this place still feels kind of like home. It's been sad to see so many people leave, and I'm not too familiar with most of the active members these days, but even so, coming back here from time to time feels like visiting that uncle you don't see very often.
The other major reason I'm still here is because of how I joined so shortly after the forums were created. After being here from the start, leaving entirely feels more unnatural than staying, even if I rarely post. Leaving now would feel like abandoning something I helped start, however small a role I played. I know I'll have to leave at some point, whether it's because the forums are shut down, or because I get a job I'm devoted to, or some unpredictable reason, I'm bound to stop visiting here entirely at some point. But for now, I still kind of enjoy being here.
As for why not many people post lately, as some people have said up above, part of it is that this place naturally goes through periods of inactivity. The Behemoth is a very small company that only releases full games every five years or so, and I think it's been a few since Battleblock Theater came out. Things should spike back up when Pit People is released.
That's not the only reason, though. I think another big part as to why activity has been dwindling is because of the size of these forums. Years ago, they received a major update that added several new sections. This felt pretty natural at the time, since the forums were growing and a few subforums were no longer adequate for the amount and variety of discussions taking place. However, this resulted in a split in the community of sorts. If I remember correctly, the update came shortly before the release of Battleblock Theater. Not many of the active members at the time were interested in Castle Crashers anymore, so they moved to the more general subforums like this one and Other Games for discussion. When BbT released, a lot of new members came but only really posted in the BbT sections. I don't know if it was out intimidation or simply lack of interest in the general discussion boards, but the vast majority of these new members didn't post in them. That puts us where we're at now, where not many people are discussing BbT anymore, and we still haven't received the new influx of users Pit People will bring.
The members still active here seem to like this place a lot, so I believe if the effort is put in when Pit People comes out, we can get a lot more activity here. Joining a new forum can be intimidating or even scary. You're entering a new place filled with people you've never met, brought together by some loose common interest that can only provide a limited amount of discussion. A lot of older members, myself included, have had the tendency to be somewhat hostile towards new members that aren't familiar with the rules, which may have unfortunately scared some people off. If everyone is welcoming in the future, we could see some more activity here.
Anyway, that's my rant. Now that I've checked in, I'll probably be gone for a few weeks.