Historically, the US had (rather expensive by the RoW standards) "unlimited" plans very early on, so most of the US users would not bother to think about the cost of SMS before sending a message. The rest of the world at the time was mostly on pay-per-SMS plans.
Then WhatsApp appeared, with an offer of $1 per year for an unlimited amount of messages.
As can be seen, the value proposition of that was tremendous in e.g. Europe, but non-existing in the US, as it just was $1 over what the US users already had. Most of the world got sucked into WhatsApp pretty quickly. The US never followed suit.
Most of us had unlimited free domestic SMS messages here in Norway as well long before iMessage was a thing, but it’s still rare (at least in my circles) to see anyone receive an iMessage or SMS. WhatsApp also isn’t widely used. The dominant messaging app is Facebook Messenger. I have no idea why this happened.
Then WhatsApp appeared, with an offer of $1 per year for an unlimited amount of messages.
As can be seen, the value proposition of that was tremendous in e.g. Europe, but non-existing in the US, as it just was $1 over what the US users already had. Most of the world got sucked into WhatsApp pretty quickly. The US never followed suit.