In the US, it's mostly a network effect I think. It was the default on half of people's phones and it managed to be good enough over time (unlike sms defaults on Android) to stay entrenched.
Also, imagine this scenario from the perspective of a non-technical user:
You buy an expensive but high quality phone that's easy to use and has a great camera and great apps. Most of your friends have the same phone and everything works great with them. Then you try to talk to someone new but they have a phone that was probably cheaper and has a worse camera and looks like it's harder to use and full of ads. Every time they message you it turns off a bunch of your features and ruins group chats for everyone. Who are you going to blame, your phone manufacturer whose product seems to work great, or this guy with a phone so cheap and buggy that it breaks everyone else's?
Thus, non-technical iMessage users see no reason to go out of their way to use weird third party messengers. It's not their fault - it's the android users' faults for being cheap.
(For clarity, I am a hardcore android phone but I'm not delusional enough to think the average person will have a better experience on Android then iOS. I had to install fdroid and a custom launcher and a bunch of side loaded apps to get a decent experience. The average person doesn't and arguably shouldn't need to know about all that)