A big part of that is that they typically don't let trolls destroy the community, or at the very least Reddit's overall design makes it easy to bury troll threads. In here, I post something replying to a troll thread and it pops up to the front page (case in point) and there is really no moderation, I've only seen 1 troll thread out of at least 2-3 per week actually deleted.
That being said, Reddit has its own problems. Namely reasonable criticism of the game will occasionally get buried as well.