They don't want to assimilate because they don't HAVE to assimilate. Why would they? Almost everything is available in Spanish: radio, TV, newspapers, magazines, ballots, driver's license tests, credit card customer service, instructions in every package, ATMs, etc., etc.
Why is it that people from non-Spanish speaking countries assimilate faster? Maybe it's because they aren't being coddled? Not everything is handed to them?
My husband is a 3rd generation Mexican-American. He hates that term because he considers himself an American who happens to be of Mexican ethnicity. He doesn't speak any Spanish and served in the our military. He is a conservative. He gets very angry when someone asks him "Do you speak English?" (Surprisingly, that has never happened to him in the 10 years we've lived in AZ - only when we were still living in California). There are others like my husband. You just don't hear from them that often because like other conservatives, he's busy working and providing for his family.