Speaking from a Catholic perspective you're going to have ethnic churches any time you have large numbers of recent (say first 3 generations or so) immigrants from specific places practicing that faith whether that's Russian/Greek/etc for Orthodox or Irish/Italian/Polish/etc for Catholics.
On the Catholic side even today in the northeast and rust belt you'll have a lot of neighborhoods where there's an Irish church, a Polish church, etc...it's not as formalized as it maybe is for Orthodox, they're all under the same banner after all, but more just the way that neighborhood has been for generations.
