It has to do with their reluctance to assimilate. I have lived in central Mexico and while I was there I did not wave the flag of the United States. I did not petition the government to change their laws to suit my needs. I spoke the language of the land. I observed and respected the customs and traditions that existed there before I came. I did not expect the citizens of Mexico to care about my needs while I was there. Nor would they have put up with it.