Earlier quoted context omitted.
Everytime iMessage is mentioned, I do a double take because it is almost non existent here in Turkey. And from what I hear, seems like most Europeans do not use it too. WhatsApp has like 99.9% market share here and I assume it is a lot bigger than anything else in the EU too. I wonder why is that though. Everyone around me has an iPhone basically and I haven’t received a blue bubble in years. The messages app is not…
As I understand it, many Americans (and all iPhones?) had unlimited-SMS phone plans circa 2009. So the pay-per-message economic conditions that caused many Europeans, etc., to switch to WhatsApp back in the day didn't do anything in the USA. Then when the same iPhone app seamlessly started sending iMessages (blue bubbles) to other iPhones rather than SMS (green bubbles), people just kept using that.
I see this listed as the reason often but I had unlimited SMS then too. In fact I remember visiting the US in 2009 and I was charged to send AND receive an SMS which was a shock.
I think the actual reason is that communication across borders in Europe is very common and those SMS's were not included in the unlimited plans as they were messages abroad. So they were subject to fees (usually high ones). I think this is the reason it was common - especially given how common it is for students to study 'abroad' in other European countries. There were a few competing apps for this at the time (Vibr I think was another but was more call focussed) but WhatsApp won in the end.