For one, the worst behaved subreddits moderation wise are often the "default" ones, meaning the ones that everyone sees when logged out. They have a favored position on the site, in other words. Every user is subscribed to them by default, which also means their content is going to wind up on /r/all more often.

Also, once a subreddit gets that size, crap mods can get away with a lot more because the volume of submissions make it hard to tell that something untoward is going on. And the people who pay attention (perhaps a few thousand on a good day) will easily be drowned out by those who do not.

Community forks happen, but never at that level. The deck is hilariously stacked towards the entrenched communities.