Might have to do with the number of people. E.g. in Spain there are fewer black people, proportionally, than in the US, and a "black neighborhood" is unheard of: black, white, people are just the same culture. Same with Asians and Chinatowns. There are gipsy communities and neighborhoods, though, with a distinct culture. And in France there are muslim neighborhoods, which I haven't seen in Spain nor the US.