I think a big reason for it is that people give up, aren't sure on how to promote their forums and etc, or they aren't willing to put time into their forums.
You probably need a lot of cash to make it work. I mean, it's like making a successful band. So, unless you're rich, it's going to take massive sacrifice.
It's possible, but now I'm trying some Facebook promotion for The Drum Lesson Facebook group (and indirectly the website) and The Math Help Facebook page (and indirectly the target site) - and it's working wonders. Especially, I'm getting tons of drummers by targeting Africa (good idea there).
Note, what I am shelling out will come out to around $200 next month - but it's I'm getting a good ROI.
It's a combination of a lot of things. Just a few of the many..
Age of the owner. The owner may be young and find a webhost to get the first month free and they work on the forum as much as they can, throw ads on the site and try to make enough money to cover hosting for the next month and if that fails, they can shut the site down.
Genre. If the forum has lots of comp. then they may see that after searching Google for other forums that relate and notice exactly how many are out there and just give up.
Experience. If the owner is a novice, they may get discouraged easily with trying to get members and staff, so they essentially give up.