Only dark skin mexicans move to the USA???

ok i went to mexico 3 weeks ago, and i was surprised to see how diverse people are down there. there are white mexicans (spanish, german, french and irish) dark skin mexicans (mayans, aztecs) and light brown skin.. mexicans who are supposed to be a mix of different races. so i was wondering why do most people in the states think all mexicacans have dark skin?? and why is it that most mexicans who move to the states are the ones with dark skin? while the others dont come here?? why do we sterotype ?? if most mexicans dont have dark skin???