From a personal perspective, I find it comes down to a matter of time...  I'm a moderator for a non-art (music) forum and, given the amount of time I can budget for forum participation, that chews up the majority. (No one should underestimate the amount of time and effort required of moderators -- particularly when dealing with forums that generate "passionate" discussion... lol).

As well, most of the time I've been able to budget for art-related work lately has been consumed in the development of items I would like to offer in a/the marketplace in the near future.  This has seriously constrained my contributions both within the forums and the gallery (something I'd like to remedy but my priority is getting the projects to completion).  That said, I do try to skim through the forums on a fairly regular basis but it's more as a "drive-by" visit and the occasional impulse buy in the marketplace. :o)

I will make more of a conscious effort to post (and probably should update/clean up a very out-dated gallery) in the future.  I am certain there are folks here with far more knowledge and experience than I, but I would like to give back a bit, having learned an awful lot from my time in here.