A lot of people of German/ Austrian heritage decided to keep quiet about that when America joined the Allies in WWI, and subsequent generations stopped speaking the language, sometimes anglicized their names, etc. For them, the process of assimilation was speeded up by general anti German feeling, but it was also a result of being a much more gepgraphically dispersed ethnic group than ethnic groups who settled for the most part in cities (Irish, Jews, Italians). They found themselves farming alongside Norwegians, Swedes, Russians, Czechs, English, Scots, French, and others, and clubbed together in Lutheran and various other protestant churches, with English of varying fluency emerging as the common language.
Also, many 'Germans' left their native lands well before German unification under Bismarck, and considered themselves Prussian, Hessian, Bavarian, Silesian, etc, and not 'German' per se. It was only under the pressure of war that this identity was foisted on them.
The Irish, Jews, and Italians were forced by economic and other circumstances (discrimination, sectarian bigotry) to stick to their own areas of the cities they ended up in and developed a strong sense of their own identity, so much so that even in the cities there were Irish parishes, Italian parishes, Polish/ Bohemian parishes - all Catholic - and marrying outside of your community was considered a betrayal of your heritage almost as serious as becoming a heretic. Most Italians came from the south of Italy and from Sicily. Jews came from all over Europe but in particular the Russian Pale of Settlement, pre-independence Poland, and the rest of the Hapsburg lands. Their experiences in their regions of origin differed, but they had enough in common and faced enough general discrimination lumped together as 'Jews' (and a good few derogatory names) to cause them to stick together.
For Catholics, parishes were the centre of social lives, with numerous activities such as am dram, musicals, choirs, musical ensembles, sock hops, roller skating in the school gyms, mothers' groups, play groups, men's groups (Vincent dePaul, Knights of Columbus), charitable drives within the community (food banks, groups distributing meals to the elderly, families with newborns, and newly arrived immigrants). The establishment of Catholic parish schools and high schools, and then Catholic universities, hospitals, schools of nursing and teacher training schools meant that they gave themselves a leg up. The Irish famously took over many branches of city services - police, fore, unions, etc, and also city governments and Democratic organisations. Italians kept their own turf too, both communities making it possible for newly arrived immigrants to find their feet, and ensuring that those who were longer established had no reason to pack up and leave.
The identities that are strongest now tend to be those of ethnic groups who stayed in cities, partly because of ties to old parishes, family relationships, and shared memories of schools, sports, and childhood fun in the old ethnic neighborhoods (though there are now many descendants of the old neighbourhood populations in the suburbs), and partly because it's easier to hold a big communal celebration of identity in a city than in a tiny town in the middle of Nebraska.