Because in US, it's just not a common application everyone has so you get a ton of "I don't have whatsapp, just text me!" from friends.

I've gotten almost everyone I know to use WhatsApp. Not switch necessarily, but use. There's only a few stragglers left. It's not a hard sell, at least in a big city where you're bound to know a lot of foreigners or people with foreign friends/family, so adoption starts well above zero.

The other replies already brought up that WhatsApp is not common in the US, but I'll also add that if your beef with iMessage is the evil corporate overlord, moving to WhatsApp kinda seems like jumping out of the frying pan and into the fire.

No reason not to except it's just not very big in the US.

Good grief, if I were ranking corporations by trust, I'm definitely not switching from Apple to Facebook of all things.