I wondered about that myself - isn't a subreddit just ("just") a forum with 1) threaded comments like HN and 2) the ability to sort by upvotes? phpBB + a few addons might go a long way to replicating that, it just takes some effort.

There's also the other aspect of combining multiple forums into one cohesive website, which mostly isn't solvable by old school forum software. For that we need a federation scheme or even just old fashioned webrings to start with.