WhatsApp is one of the most popular messaging apps. In the early days, BBM (Blackberry Messenger) was used by everyone but was limited to Blackberry devices. WhatsApp provided a free messaging platform for Symbian (still popular in emerging markets), Blackberry, Android, Windows Phone and iOS. It was one of the first apps to be truly cross-platform, not just iOS and Android. With Symbian and Blackberry widely used in emerging markets, it got a lot of market share. Dozens of messaging applications have emerged since then, but there are two important aspects to such an application: (1) mass adoption by networks of people, and (2) being truly cross-platform (not just Android and iOS).
When using a utility application for Mail or IM, its easy to switch to a better application. With messaging, it does not make sense to switch when five of your contacts are using it versus hundreds on the other platform. So people are not likely to switch. What developers also forget is that Android and iOS are the major OSes, but only in North America. Asian and South American markets are still dominated by Symbian and Blackberry devices with Android gaining market share rapidly. Also when messaging patterns for WhatsApp is analyzed, its heavily used for international communication. When all of you contacts are in the US, you could use texting as most carriers now offer unlimited free texting. But for international messaging, apps like WhatsApp are used primarily. This means that supporting platforms in those markets is as important as iOS support.