Lack of time would be one of them. I would imagine if I could afford more things I could afford more time as well and have a better outlook on life. That being said I honestly only ghost 3DS Central and my accounts for other forums have been dormant.
The lack of content I find interesting, or just lack of content in general, are generally what get me to lose interest in a forum.
There were a couple I left due to toxic users. Either by douchy power tripping mods, or one had a really prominent user that basically hated everything on top of being a douche. It's not fun talking to someone who just has nothing nice to say about... anything.