Maybe because SMS became very popular in the US, and iMessage originally was just an extension of SMS from a UX perspective, 2 years after iPhone launched. It added functionality and fixed issues with SMS (multi-part messages, images, etc). This led to the dislike of "green bubbles" because it meant you were forcing a degraded experience on the other person. That started a positive feedback loop for iMessage.

My understanding is SMS never got anywhere as popular in the EU.

On the contrary, SMS was insanely popular in Europe before the smartphone revolution and before data plans were cheap just that people didn't want to lock themselves in with Apple's proprietary iMessage otherwise they couldn't talk to others who don't have iphones.

The Europeans tended to lock themselves into Blackberry Messenger for the same reason Americans initially locked themselves into iMessage.

At the time, the alternative was to either pay a per message fee or pay monthly for an unlimited text messaging plan.

I never met teens in Europe who wanted to own a BlackBerry. Nokia were the craze then iPhones took over.

Did you not know a single teen in the UK during that period?

Unlimited messaging without extra fees was quite a draw.

Unfortunately I didn't know any teens in the UK

Anyway, UK was/is a lot more aligned to the fads in the US than in continental Europe to which I was referring to since the UK is no longer in Europe ;) j.k.

Blackberry was never as popular in Europe as it was in the US.

Also unlimited messaging plans back then weren’t a thing in Europe yet either.

SMS was huge in the UK and Ireland from the early part of the century, and the operators absolutely gouged kids with their topup pricing.

Because it's just there on devices they have. It is the default for tweens and teens who just got an iPhone handed down from a parent, and it works on day one. Regionality happened with IM clients back in the day as well - some areas favored AIM, some Yahoo, some MSN. Why did that happen? It just does.

That's called network effect.

I think it is because US was the first to get "free" sms long ago. And in the rest of the world chat apps (viber, whatsapp, telegram, messenger etc.) where a way to get free messaging.