As far as I understood, this is because US carries throws unlimited SMS for free and data is costly.
In my country, it's possible to get unlimited sms but for additional fee on some carries (other carries just provide N SMS per month) and data prices are rather low even now.
If they're on Whatsapp, then they've got their number. Surely everyone can receive an SMS? If its an emergency, you're not going to care what app you're using, just so long as you can communicate.
While SMS would be great for low-bandwidth use cases like this, most people don't use it to the point where having contacts accessible with it on the other end would be an issue.