I honestly don't know why WhatsApp is so popular!! I know it is cross platform but so is Viber and other apps. Is there anything WhatsApp does does better then Viber? I would quit using WhatsApp anytime if it wasn't for more of my contacts using it compare to Viber.
First-mover advantage and network effects. The novelty of sending text messages over the Internet based on phone number, combined with a wide availability on various systems. Once the snowball starts rolling more and more users will get it, because their friends have it too. With more users, Whatsapp becomes even more important. At that point it becomes the de facto standard, regardless of whether there are better alternatives out there. If the app works and does what it should do, then competitors will have a hard time competing with Whatsapp.