There is: discoverability. If you search in the App Store you’re going to find those two versions and then they’ll have to prominently explain “this is the version of the app you can look at porn with” and “this is the kid-safe version”. They’d probably choose different wording, but that would be the message. And you don’t want to advertise adult content on your platform in any way, that just doesn’t end well.

It's also highly questionable Apple would let a "full" version in the store tbh, as it's only purpose would be NSFW servers.