Presumably because they are in the shadows

Mexican Immigrants Do Not Assimilate Quickly in US, Study Finds

Immigrants to the United States are doing a good job of assimilating, with immigrants from Cuba, Vietnam, and the Philippines leading the way in adapting to an American way of life, according to a new study. But the one group not assimilating well is Mexicans, apparently because so many of them are in the country illegally.