Moderation and content quality.
You know the reason why people prefer reading the New York Times instead of checkout-counter tabloids? The reason why people like buying from Amazon instead of Ebay? I mean, Ebay usually has better prices. Why would anyone buy from a store just because it's able to (or at least claims to be able to) filter bad products and keep off scammers?
That's Mastadon's advantage. The userbase isn't crap. Twitter, so far, has shown no indication that its userbase is ever going to be anything except crap. I don't think it's possible for Twitter to even start fixing that problem. Moderation doesn't scale.
If you're on social media not just to connect to friends - if you're on social media because you want to actually consume cultural content or interact with new communities, then this is like comparing Youtube and Netflix. 90% of everything on Twitter is garbage, and it's suddenly your responsibility as user to sort through all of it?
People want a platform that just works. They don't want to have to download custom block lists off of Github, or turn off DMs whenever anybody gets mad at them. They don't want to have to curate their own timeline. They want somebody else to do it for them, and Twitter can't do that.
> Alternatively, is there some new and novel porn that Mastodon will allow me to access?
Probably? Twitter has policies about pornography - they're inconsistently applied, but they're there. You could make a dedicated Mastadon instance that allows whatever. You can also choose not to have porn show up when you search across an instance because, again, Twitter can't moderate and Mastadon can.
I would feel a lot more comfortable allowing a kid to sign up for a Mastadon instance than Twitter.