Well, I am not an linguist per se; if I were to turn the tables around and ask you why some tribes in Northern Africa are speaking German...what would be your answer!?
Furthermore where are you getting your stats from?
They are all over. Start with looking for Nahuatl language on Wikipedia.
Mexico is filled with Indian tribes, Aztec related tribes are still there. 1.5 million still speak Aztec/Nahuatl but many more of the same stock speak Spanish.
Mexicans have more Indian blood then European. The upper class is mostly white.
"Northern Africa are speaking German...what would be your answer!?"
Vandals?