iMessage is the stock messaging app in America, and America is the driver of the tech industry, so it stands to reason that Google would focus their efforts there.
It has been my experience that WhatsApp was relatively popular in the US initially, and had a major drop in popularity after meta bought it. Americans seem more aware/sensitive to privacy and data harvesting than in many other countries.
Perhaps the market penetration that Apple had in the early days of smartphones has additional impact on iMessage uptake vs others. I remember using my 1st generation iPhone internationally and many people had no idea what smartphones were even though Nokia and other manufacturers had some functionality. From what I recall.., it wasn’t immediate that iMessage had evolved beyond sms. Although once it did Apple enabled much more robust messaging without the tether to antiquated sms cost structures.
Although it is likely that sheer wealth has been the largest driver in America. In many countries cheaper smartphones are far more prominent, and WhatsApp has become ubiquitous as a lower cost communication platform than traditional sms.
In my case I’ve just always used iMessage, and for the few people I need to communicate with outside of iMessage I source the current best alternate solution. I’ve never used Facebook messenger, and I avoided Blackberries for a solid decade. I typically use Signal at the moment, although sms is perfectly fine for most of my non-iMessage communication.