I think someone already noted that sms is simply the most reliable and in many countries that are not bound to contracts etc not all users have data plans and rely on WiFi free hotspots or work/home ones.
Whats App is prevalent here and in Thailand Line is, which also has a desktop App for Windows, Line I found useful at times were free WiFi calls has got around poor cell reception.
Imessage maybe fine in US with a high % of IPhone users but soon dwindles elsewhere hence the popularity of other apps.