there must be a bit more to it than that. AIM, Skype, and several others were viable options with existing userbases.

Once upon a time in a messenger landscape far far away there lived a king called XMPP. It had a lot of powerful children, WhatsApp, Facebook Messenger, Google+, and even Skype amongst them. And they all worked together in a big federation towards the commonwealth of all, freely sharing their metadata. But then some of the children grew greedy, jealously guarding their own gardens behind higher and higher walls, breaking down the federation. And thus the era of the warring messengers began. But prophecy foretells of a prince to unite all the disparate standards in one big Matrix again, completing yet another revolution of the XKCD 972 wheel of time.

For real though it was phone numbers. WhatsApp always worked based off of phone numbers, which is an identity confirmation method that was immediately familiar to most people at the time, even more so than email.