What's special about Whatsapp that made them choose this platform rather than (say) email, any other chap app, or even Facebook itself?
Also, it was free and works on Android (Very important in South Asia).
Many older people get smartphones just to use whatsapp to communicate with others. I'm not joking. I upgraded my mom to a smartphone against her will so that she can use whatsapp to communicate with friends and family more easily.