Spammers and scammers. People are constantly trying to use any service for illicit purposes. A somewhat-easy way to deal with that is to just ban them. Because

Spammers and scammers. People are constantly trying to use any service for illicit purposes. A somewhat-easy way to deal with that is to just ban them. Because an email address is used as username and an email address usually maps 1:1 to a person, this means you just got rid of a Bad Person. This breaks if access to new email addresses is trivial. You now have to invest actual effort into validating that a new account isn't someone trying to do bad stuff. Or you can just block all the services providing easy access to email addresses and outsource the issue to big providers like GMail and Apple. It's a lot less effort, and you'll lose near-zero legitimate customers...