I think the real reason is that in the early 2000s, pre-iPhone, texting was ridiculously expensive in the US and much cheaper in the rest of the world.
Also, Americans were using awful phones while the RoW was using much better phones like Nokias thst even in their non-communicator series made texting far easier thanks to stuff like T9.
So the rest of the world used SMS and texting far more than Americans Pre-smartphone.
When smartphones came out and WhatsApp was released, the RoW was far more willing to pay the dollars needed to buy WhatsApp (it wasn’t free before FB bought it) because they would easily save on their low but still not free SMS charges.
Whereas Americans were not as willing to pay for an app to do something they didn’t do anyways, and anyways by this time phone carriers in the US made SMS free for the most part.
As a result WhatsApp became entrenched in the RoW well before iMessage was released (2011), so non Americans continued using WhatsApp, even as Americans started using the free available iMessage instead.