Network effect. SMS works everywhere, all phones support it out-of-the-box. WhatsApp is opt-in. Almost nobody I communicate with regularly has a WhatsApp account.
Personally I have one person in my contacts who's not in the US. This is someone I communicate with about twice a year, and we do indeed use WhatsApp for that.
[0]: https://en.wikipedia.org/wiki/List_of_country_calling_codes
But as other comments mentioned, the number of people I "text" with regularly without a U.S. phone number / the ability to send and receive SMS is very small.