General submissions have tons of comments, so there are actual discussions going on, motivating users to check back often. Also (at least for now), the discussions have less noise.

Content-based subreddits (like instantkarma, holdmyfries) where there is minimal discussion can be easily replicated with a bot, until organic submissions reach a critical mass.

That leaves community based subreddits, but when Reddit aggravates the community leaders they can easily move (like piracy did).