The blue bubble is the essencial first world problem
From what I've read elsewhere, the main reason was SMS charges: many years ago, it cost money to send SMS texts in most countries, while in North America the telecom companies were faster at offering free texting (or at least flat-rate). So elsewhere, various chat apps arose and caught on: LINE in Japan, WeChat in China, KakaoTalk in Korea, WhatsApp elsewhere. Now that everyone is used to these services and they've become so popular to the point where they're basically mandatory to participate in society, why would anyone want to go back to SMS? The only problem with the messaging apps is that they're all separate from each other so they're silos, but unless you have a lot of friends spread across different countries, that's not an issue. And these apps don't disciminate based on which phone you own.
More than anything I believe it's more related to social circles than specific countries. I use Signal, but in frequent, because that's what colleague around the world use, and I have two friend on Signal, but that's it. It's not something I consider standard. I know people who use WhatsApp pretty extensively, but again only within a limited social circle. A lot of people use Facebook Messenger, but I've never seen anyone expect that I have it installed. SMS is still the default, and sometimes it just happens to be delivered via iMessage.
Messaging software is fully determined by who you want to send messages to. It tends to unify across groups that interact, so knowing someone's nationality will tell you their messaging software with high confidence:
China: wechat
Taiwan, Japan: line
India, Australia: whatsapp
Russia: telegram
USA: facebook messenger
and so on.
Especially when Americans talk about Europe "not using iMessage" it is frequently assumed that we're on WhatsApp, Signal, Telegram, Viper or something like that, the sad truth is that Facebook Messenger is probably the biggest platform in many countries and in top three in the rest. That just not the platform that those most critical of iMessage wants to be on either.
My chat network is pretty global and iMessage, Signal, and WhatsApp seem to cover all bases in practice.
Solution: another proprietary service
This is just Internet Explorer again, disguised.
I'm just sad Apple didn't push for iMessage to become THE messaging standard. Google is still trying with RCS. Hopefully it takes off.
A proprietary messaging service is fine, it was what people liked about Blackberrys back in the day. The silent upgrading is the problem.