Its a clash of cultures too. Other communities, particularly those that are largely a migrant population, tend to be closer than, say, communities that are not. In larger cities, there are still sections where there are large ethnic population. This brings them closer, through culture. Whereas, in mixed communities, there is less mingling-likely due to different cultures. We all take comfort in the familiar-this extends to our micro communities.
Also, as was said above, I think those coming from an impoverished nation, or at least the impoverished part of another country, have a larger sense of self-worth, and more of an adaptive attitude-simply because thats what they need to survive.
Mind you, these are my opinions only, but I am interested now in perhaps the demographics on how different sub-cultures within the Katrina disaster area fared-who left, who stayed, what level of damage was done to their area, what sort of help they received, how they dealt with it day in, day out.