Not to generalize myself right under a pile-on, but back in the early 20th century, America was a melting pot and everyone understood that. People came here to become Americans. Nowadays there's something unsavory about that. Ethnic pride (ie, transcendent tribalism) demands that an individual resist assimilation. For many immigrants today, America is a network of troughs---and sewers.
The concept of the "melting pot" is actually an invention of the 1920s. I do agree that the emphasis on muliculturalism and the glorification of ghetto culture has been highly damaging to the United States.