I get where you're coming from, but I really don't think this is the case. I can churn through 400 reddit comments quickly, but 20 pages * 20 threads/page with [bulletin board software] is nothing short of a pain - not to mention the quote within a quote within a quote and tons of wasted space for avatars/signatures, which can only be disabled on some forums. I can see the benefit for long-running events (a reddit thread over the course of a few weeks would be hard to read chronologically) but I think there's a lot of room for improvement, much of which is probably up to the designers as much as the developers.
Also, I think the barrier to entry in a 400 comment thread is largely different as well - with reddit, the issue is whether your comment will receive any visibility, whereas, IMO, in a [bulletin board] thread, catching up on 400 comments of discussion seems daunting and almost pointless.