The other problem is that several subreddits were known to "leak," where the subscribers to some subreddits would go out and spread that kind of thing across the rest of the site. If the racist content stays in their subreddits, it's still terrible that it's there, but at least it can be firewalled off. But when the users of those subreddits start spreading that content through the rest of the site, it's much more difficult to avoid it.