I guess because Whatsapp provides a unified and standard way to communicate between people on different provider networks / countries for free? You get groups, text/images/location sharing, video/audio calls, cross-countries, that works very reliably, and you only need internet (which can be found almost anywhere even if that's just wifi).

MMS still don't work reliably for me (lost a message just last week that someone swear to have sent). And for a long time they were not free as well.