By that time SMS were free and unlimited under most plans in the US but not in other places. WhatsApp, Viber, LINE etc. caught the non-US markets rapidly as a free alternative to an expensive service. These messaging apps didn't do as well in the US, as they had an equally free built-in alternative (SMS).

Later, most operators made SMS free everywhere, but it was already too late.

10 years later, we still see the same results.