I think it initially had to do with the fluid self-regulating style of the forum. Quality posts got up-voted into more prominence and the drivel got downvoted away. This worked well back when it was new, but now it's all just manipulated by brigading and content manipulators, and anyone with a smartphone can try to "outfunny" the last post.
Anyone ever heard of the Eternal September? The phenomenon very much applies.
»
en.m.wikipedia.org/wiki/ ··· eptemberOne forum that has remained quite high-quality is HackerNews (»
news.ycombinator.com) which actually has some fantastic discussion on various topics (not all tech-related). But there aren't many places that I know of, apart from niche communities that remain clean because of the self-limiting nature of a niche hobby topic.