Why can’t I have a different username for Mail and Social?

You can do this with two accounts. For example, register basic@librem.one for chat and social posts, and register complete@librem.one for mail and tunnel services.
Multiple accounts also allow you to implement a separation of concerns. For example, you could have a business account, and a separate account when discussing a medical condition with your support network.
To protect your privacy, we do not store a mapping between accounts, except in the case of family packs where one user is financially responsible for multiple accounts.