It's a very good question, and the truth is that it's one we've been struggling to find an answer to for over a year now. When the site started out, we only really had to compete with Bungie.net and Waypoint, and that wasn't difficult since, deserved or not, they both had reputations for heavy-handed and unaccountable moderation: all we had to do was let people know that they could speak their minds freely here and, provided they stayed away from profanity and abuse, they wouldn't be on the receiving end of warnings or bans.
Now, however, we have to compete with /r/Halo on Reddit and TeamBeyond, and that has a lot of advantages over us. People want to feel as though the discussions they're having are being noticed by the devs, and while you can be noticed by and chat with community managers and even the pro team elsewhere, that's something we can't offer. In addition, TeamBeyond gets regular advertising from 343i directly and is probably the largest discussion hub for its pro players, while since everyone has a Reddit account these days and it has such a huge potential audience, it's really easy to just jump onto /r/Halo or /r/HaloStory or whatever and start posting from an existing persistent profile. Put simply: we can't compete with the sheer numbers of Reddit, and we can't compete with the access, recognition and official 'legitimacy' of TeamBeyond, and as a result we're struggling for activity.
It's a sad truth, but once you start getting big businesses/corporate entities and official channels the smaller, independent places suffer as a result, and sadly decades of economics have shown us there isn't really an answer to that beyond finding money and coming up with a unique selling point of our own. We deliberately don't run adverts on the forums or YouTube because we want the user experience to be as pleasant as it can be, so whatever money we use needs to come out of our own pockets (and while we're willing to do so, we're a volunteer staff mostly of student age so we're naturally quite limited), and it's very difficult to think of anything we can do that other places either can't throw more money or a bigger audience at.
The good news is that since we've always been very community-oriented, so long as we have members who keep wanting to come here then the staff and forums will be around for them, and since our active member pool is so small you get to know everyone personally really quickly. We're also really lucky to have a partnership with Zandril, for whom we're hosting the official submission forums, and the i01 team, whose game is going to be something really special once released (my jaw practically dropped at what they've been able to accomplish the last time I had an update!), and we have plans for a potential competitive league at some point in the near future, so we're not completely without advantages. But for now, the best we can do is to keep producing content to give people a reason to log in every day, and so while that's not a very good anser, the truth is that so long as we're struggling for money and members, it's the only approach which is really practical.