As someone with a website, but no forum, I'd say it is really hard to find people, wanting to engage with your specific forum. Putting in a lot of work just to leave it abandoned doesn't make much sense. Also, my niche is True Crime, I don't think I can get many views with that, let alone commenters who actually discuss topics without moralizing. If it was a personal site, it would be even more difficult, because what on earth should I open new topics about?
What I was gonna say is; I'm happy if I get a message in my guestbook it's a rare phenomenon. I can't expect people to register to a dead forum, but if there were people wanting real discussion, I wouldn't mind creating a forum.