There’s legal issues involving hosting, and that includes mirroring, or acting as an aggregator for, porn. Many admins simply don’t want to deal with the headache of that.
It’s fine if the fediverse is (mostly) split into sfw and nsfw sides, after all, most people had different accounts for both on reddit in the first place, and much of the rest should have.
And it’s not like admins are defederating lemmynsfw.com – they’re only blocking nsfw posts, lemmynsfw wisely has a strict “tag everything” policy. Which means that if someone wants to use their lemmynsfw account to engage in discussion on the sfw side, nothing’s stopping them, as long as it’s sfw.