Most people in the world don't (or barely) use email, specially in EMs. Also, you can access you email from many different places, you (usually) can't receive an SMS on the same number from many different places.
Another advantage of using phone numbers is that people actually have other people's numbers in their phone address books, which whatsapp uses. Very few people have their friends' email addresses on their contacts.
Another advantage of using phone numbers is that people actually have other people's numbers in their phone address books, which whatsapp uses. Very few people have their friends' email addresses on their contacts.