This isn't really true. WhatsApp was used pre-acquisition and continues to be dominant throughout LATAM, Africa, and Europe in addition to US/NA. Only in the APJC region and Russia do we see significant divergence in messaging apps.

Having traveled extensively in these places, I always theorized it was due to UX behavior aligning well with the local languages. While the countries WhatsApp dominates speak different languages, they all use the Latin alphabet. In Russia and APJC there are many non-Latin alphabets used and those languages may also use different directions for writing/reading than Romance and Germanic languages.