[fidor]

One of the reasons, at least here in the UK, is that for a long time you only got so many SMS messages in your phone contract. MMS was also awful for a long time. WhatsApp solved both those problems, and most importantly was on every device possible.

[thirdsun]

And it was easy to set up. No need for accounts and email addresses. The fact that your phone number served as id and login meant that even my tech-illiterate mother was able to use it.