I don't understand why they don't allow different domains. "gmail.com" is running low on email addresses; if they added more domains, they'd be able to really scale up their email offering.

Branding and marketing. It makes it crystal clear how popular it is.

And gmail.com isn't "running low" on addresses, I don't even know what that means. Whatever TLD you'd prefer, just append it to your username instead. Exact same amount of uniqueness.