NYC and LA are culturally far closer than Germany and Greece. For a start, NYC and LA speak the same language.
The whole "different language" thing stands in the way of this. Plus, the Okies never had to deal with Californian "nationalism" in the 1930s, while, on the other hand, if you're Polish, Romanian or Bulgarian and you've gone to Western Europe in search of a better job you're always looked at as a foreigner, a guy who "has come to steal our jobs and destroy our identity".