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!
I guess in reality, a NSFW instance could probably also host SFW.
But it’s less likely that someone running a SFW would want the additional headache of hosting NSFW.