1. If you need to reset your pw at any point, we need either your email or phone number. We don't ask for your email. A phone number is most likely tied to a real person.

2. It's a unique identifier that allows you to add your existing contacts more easily. It's much more convenient than trying to figure out the usernames of all your friends and family.

3. It's harder to automate creation of users by requiring a valid phone number, versus usernames (and potentially even email).