Earlier quoted context omitted.
In the U.S.A., people use iMessage by default, because they use text messaging. People don't use texts in Europe (we use WhatsApp and similar, but WhatsApp by a long margin). The reason people still use texts in the U.S.A. is up for debate, and it's something I've thought about for a long time.
I think it's a historical artifact -- by the time that iMessage launched, the US was well into the phase of free, unlimited texting, so there was no barrier to "just texting". Since there was also no barrier (in terms of costs, subscriptions, or installation friction) to using iMessage, it was a drop-in upgrade of the user experience for SMS. In contrast, I still have to pay a fee to send SMS messages to my European…
So people had no real reason to move off of texting, because it just worked, and Apple's iMessage just looks like texting seamlessly.