The fact that WhatsApp is also gigantic in large countries with no intermobility between them (Mexico, Brazil, Nigeria) would suggest that the only issue that mattered was the fact that ~2011, mobile carriers would charge insane rates for SMS.
For sure, but from what I understand, in some European Countries in 2011, SMS was free if your recipient was inside the country but cost if they were not due to tariffs and interconnect fees. Since Europeans do move between countries thanks to that European Union thing, it became a problem and WhatsApp became the solution.
Sure, other countries it might have picked up domestically thanks to high SMS prices but low data prices.
