Mostly, because they are proud of being Mexican and they have a long history backing their beliefs; also, the language barrier (most of them speak Spanish), religion (Roman Catholicism) and culture...
Nativists are people whom do not like and usually discriminate against immigrants. Therefore, they feel as if immigrants will not be able to truely assimilate into the new culture that they have...
Yes. A hyphenated-American is a non-American. If their former culture is so important, they should return there and leave America to those who choose to join and become an asset instead of a dividing...