That's still different servers, though, which might even be banned on your home server. Meanwhile, Reddit just has it as base functionality on the main site.
The reason for this (imv), is that hosting for general and nsfw communities are very different kettles of fish.
For general communities, you want lots of consideration, nuance, and nudging when moderating and setting what is acceptable content. As this helps to gradually build positive self regulating communities.
For NSFW, you have to be hair trigger deleting stuff, or you just end up hosting horrible shit.
I have respect for the guys willing to deal with hosting NSFW content, because it's more than I'd ever be comfortable with!