A forum must have a reason and an audience.
Let me elaborate.
A forum must fill a niche and have a good target audience or topic to keep going. To compare a forum about Zelda and a forum about Loneliness is a good example. People can only talk about Zelda for so long, really, and they're probably more likely to find a general video game forum to discuss it on. This forum, on the other hand, is one of very few for the audience it caters to. I haven't really found anywhere else where I can come talk about my problems or just BS about my day to day life from the perspective of someone lonely, and have people relate to me.
Subsequently, the more of these forums crop up, the less successful they will be. I'm established here, I like it here, I'm not going elsewhere unless this one disappears. Think of it like a cafe: There's a restaurant here called AJ's, barely more than a hole in the wall where I like to go and have a cup of coffee and a cinnamon roll or maybe breakfast, and they open really early. The waitresses and the owners know me by name and know I need frequent coffee refills. They know I will almost always order biscuits and gravy. I might try other restaurants, but it would take a landslide to make me start going and having coffee there instead of AJ's.
A forum must be accessible.
This forum is accessible because it shows up easily under search engines and it has a dedicated '.com' domain; bjarne has been good to us by not invading us with advertisements. MyBB is also a pretty simple, clean forum to use. It's easy to go create an EZBoard or something for a topic, but it's difficult to find even with a good keyword search, and then if you do find it you're plastered with pop-up ads.
Beyond that, simple time is necessary. A lot of your initial audience usually disappears...when I first got here and looked through the early pages of the board I noted that a lot of the people made maybe one or two posts. It's a lot like a Stalactite in a cave; it forms slowly from a series of water drips, and most of the water runs off.