I mean, as I understand it, prior to WWI you had all sorts of thriving German-American and Irish-American and so forth communities, and now you mostly have a bunch of people running around like "I'm German!" and "I'm Irish!" without even knowing about the cultures, much less speaking the languages or having actual traditions. What kinds of things happened to mostly get rid of these kinds of communities? (I know they're not completely gone by the way, but they seem to be drastically reduced.)
Germans were unpopular because of the war so they cut out a lot of traditional German celebrations, things like beer gardens, spoke English and some even changed their names.
Plus the US ended wide open immigration about that time. Of course there was no immigration because of the war, but soon after Congress changed the immigration policy imposing quota numbers much lower than when there was no quota.
Immigrant cultures never completely died out but as people became assimilated and the older first generation immigrants died off, it became hard to tell their children and grandchildren from any other American.