That explains it, I was actually going to mention New York as a special case before I changed my post to ask that question first. New York and London are the two most diverse cities in the world, there are whole districts of each city that don't speak English. In Minneapolis, MN the light rail uses four different languages for instructions, two of them are dialects from Sudan. All the big cities in America will have sizable populations that speak only enough English to get through their daily lives, but their children, who are speaking English in school and their parent's native language at home, are speaking English most of the time just fine.
There are kids who speak with a southern accent too, it's all in what they're around. If I go down to Texas for a year I'll probably come back speaking with a drawl.