I think there's a variety of factors that have contributed to the downfall of forums in general and this site's forum in particular.
First is that it's 2013. Nobody really uses forums anymore. They're a relic of the late 1990's and early 2000's. The technology's outdated. We've got all these other sites like Facebook, Twitter, instagram, where you can interact with people you are already friends with. Why would the average person come to a forum full of Internet strangers when they can talk to their real friends? Obviously there are exceptions as most people, like most of us I'm assuming, use forums as a supplement to social media. But for the majority of people interact on those other sites is enough.
Second are the unique problems Supercheats faces in particular. With the advent of Achievements and Trophies cheats kind of went right out the window, besides in rare games like GTA. Now, instead of being able to discuss cheats for games on the forums people can only discuss the games themselves, and there are often much larger communities that focus on those games. It's not Supercheats's fault, that's just how the industry developed. If you look on the Latest posts you can see it clogged full of mobile game forums with people asking for hacks, cheats, and god knows what else. Some of the stuff posted probably isn't even legal.
Rich has tried several things to kind of jumpstart the community, and he's had varying degrees of success, IMO. The biggest of those is guest posting, which is good and bad. Good because it promotes activity on the forums, bad because it isn't exactly the kind of activity we'd like. A thousand posts a day of "100000000 gold plzzz" isn't exactly what we'd like to see. The other thing he's done was cutting down the staff, which I think was a success. Nobody seems to have a problem with any staff members, or at least I haven't been notified of anything.
KingofCorn pointed out that most people here visit solely for cheat codes, and I'm not sure that's true anymore. I think more people now visit for the big game guides and walkthroughs, but obviously I don't have any data on that. Rich has kind of shifted the site away from traditional cheat codes with 3 things: the guides, videos, and wikis. The guides seem to be successful, as we keep getting more of them. Videos and Wikis, not so much. The videos are a nice idea, but there are just so many more places to get the official content that's put up, and I'm not sure many members submit videos. I'm not sure how successful the wikis have been either, as thwy're purpose is to be built by the community, and I don't think the community (or lack thereof) contributes at an optimal level.
Excuse the essay, but this topic comes up a lot and I'd just like to put all my thoughts into one post.