Northern Light
Superstar
It can be neatly summed up as: we don't border a third world country.
The difference in economic standard of living for many Mexicans is certainly a big part, and once a much larger part of the U.S. immigration situation.
However, as I noted, racial tension in the United States is not exclusively or even primarily about the Mexican expat community in much of that nation. There are other tensions at play.
Those too, in many respects center on access to economic opportunity, but that is an internal U.S. issue rather than an international one.