Multi platform, mod tools for groups, broadcast only groups, etc...

The correct answer is it's not better in features. Meta subsidizes the data usage through whatsapp in Latin America and so people started using whatsapp en masse because it was free. By now the network effect is so huge that it's hard to topple.

just first mover advantages and net-effect

This is one of the most common reasons I've heared, but not only for Latin America. I've heard it in Europe and Asia as well.

iMessage is iPhone only. And the User Interface of threads in groups is horrible.