In a nutshell, one of key aspects of the reddit system is that an upvote arrow affects everybody equally, so the majorities affect whether something is negative or positively rated. When you take that into account for where and whether you show a post, the net effect is that at scale you pretty much guarantee that the majority within a given community can suppress the minority. The majority may not even have to necessarily try; it's something built in to the system, even before we get into matters of people deliberately trying to game the system.

Systems like Slashdot, HN, and Reddit try to fix that either with human moderation or a variety of heuristics to try to minimize the impact ("that looks like a voting ring, let's make its votes worth less", etc.), but another alternative is to make the upvotes not be universal anymore. Instead of an upvote meaning "I think everybody should see this more", it could be rewritten to mean "I think I want to see more of this poster". It's more math, but we have bigger computers now. As a side effect, it's robust against a lot more attacks, though I don't guarantee it wouldn't have other weaknesses.