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.
See also http://slatestarcodex.com/2015/07/22/freedom-on-the-centrali...
Usually it's easier to just make a new subreddit and convince people that yours is better.