Generally speaking, it depends on the niche, or a new owner wants rapid growth on their online community, or they don't have the passion to run it.
They are many factors that kill the interest in a new forum quickly.
However, I've seen many forums over the years that were shut down due to the lack of dedication from their staff team, and from their ownership as they wanted to make a quick buck from their forum. When that didn't happen, they closed their online communities down.
Forums take a long time to build, they don't grow over night. If you're a forum owner, you're in it for the long haul. It is not something that'll grow with one simple seed, it is something that'll harvest its growth within time.
Weak moderation issues can rapidly hurt interest in a new forum as well, but it isn't too often.