I've often wondered this myself. To me this seems like a great tool that many people do not take advantage of. I've created a site based pretty well on that idea and it amazes me how difficult it is to find people that want to participate. A majority of my visitors come from search engines, its may even be difficult for people to realize that theirs even a forum on this site.

I have a similar thread on the go, see http://www.refinishnetwork.info/index.php/General/525-Suggesting-Online-Discussion-to-Other-Techs.html

I have never had any tech that I've suggested this site to in person actually come participate in the discussion. Their seems to be a lot of ego's in this trade and people don't seem to think its worth their time. One interesting thing that someone pointed out to me too, is that many people are intimidated by computers in general. I still do get many good techs and owners across the world that enjoy the discussions.