An anecdote from South Africa: SMS was charged per message at very high rates. MMS even higher. Mobile data, while expensive too, was much cheaper per message. Also, free wifi was quite common in many urban areas. All this at a time when WhatsApp was much much simpler and easier to use, with good UX. The person to person advertising for WhatsApp at the time was “it’s free”, which while not 100% true factoring in mobile bandwidth, it was effectively true since mobile data and WiFi was at least 1 or 2 orders of magnitude cheaper than SMS or MMS.

That stayed true for a few years, and so WhatsApp became the de facto standard for messaging.

I can’t speak for other countries but in South Africa that’s why. And the rest is just networking effects.

Ah, that makes sense. I'm fortunate enough to live where we have unmetered SMS/calls/data so I've never had that issue (at least not while WhatApp's been around).