Someone more knowledgeable can correct me if I'm wrong, but the diversity of Mexico might have to do with how strong the Central American nations were at the time of colonization. They had huge cities and much more complex government systems than their Northern native neighbours. There was thus less massacres and more intermingling down there.

The modern distinction may have been caused by the border itself (e.g. if you are a new immigrant in US, you probably have better opportunities away from the border).