I'm still an active user in two different "crappy" old forums where the posts are shown in sequential order.
Let me tell you one thing, after having used reddit/hackernews tree-like structure for years, it is hard - extremely hard - to go back to using the older types of forums.
The problem is that they need very active moderation. It only takes two users to completely derail a thread, or clutter up everything. When you have two users that start arguing / discussing, it completely ruins the flow for everyone else. Also, jumping back and forth from page to page is annoying.
That's the logistical aspect of it.
Most older forums I used, seems to have migrated to facebook groups years ago.