My personal take on this is that it's the dark side of multiculturalism. The loss of the melting pot as a cultural meme was probably a sign of this. Social cohesion depends on a set of shared values. The "melting pot", for it's issues, provided the shared national myth to align and unify the disparate cultures.

Take that away, and you get the natural result of multiple cultures with drastically different moral codes, and no core to form around.

Interesting point on drastically different moral codes. I'm curious if, during the height of immigration to the US, the "melting pot" tended to consist of people immigrating from areas with mostly similar moral codes.

It appears that many immigrants retained much of their culture while still integrating in ways that let different cultures work together, but maybe that is because the moral foundation was similar enough to allow cohesion as a whole while retaining some sense of cultural identity.