Many Americans with international contacts do use WhatsApp, or even Americans who travel internationally.

However, because many Americans never leave the US, and neither does anyone in their social circles, many Americans never had to deal with international messaging and call costs. Hence they went from unlimited SMS to unlimited iMessage, once Apple rolled it out.

However, if you traveled internationally during 2008 to 2012 or so period, then WhatsApp was a godsend, and almost everyone I knew (since pretty much everyone in my social circles vacations or travels for business internationally) has WhatsApp.

But everyone pretty much has iPhones, so the preferred chat is iMessage due to the high image quality, but if someone does not have iMessage then we switch to WhatsApp.

Probably many reasons. One reason is that SMS was already ubiquitous, and it doesn't require an additional data plan the way 3rd-party apps do.

Why did the rest of the world adopt WhatsApp and alternatives? I don't think they were any later to text messaging than the US..

There’s no way in hell I’d voluntarily lock myself into a Facebook service. WhatsApp’s a non-starter.

Because proprietary messaging apps are sad and unnecessary.