By using this logic, WeChat should also dominate USA - which is not happening. It's still interesting to me that WeChat is not more popular, considering its superior (for me) experience over WhatsApp.
One person has several social connections, at church, work, gym and so on.
Your contacts have their own contacts and so on.
At some point, this network crosses borders. Being a country with so many migrants, this happens real close to you at USA.
If a Brazilian living in USA wants to talk to his family or friends in Brazil, he must use the same channel everybody here uses (or he will felt excluded).
I don't know the numbers, but this can indeed happen with WeChat, but I think whatts is used in more countries.