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.