While German ancestry is still the largest single ethnic heritage group it isnt necessarily the most cohesive one. Much of it is either centuries old pre unification or heavily scrubbed in no small part thanks to the World Wars.

While despite the large numbers of migrants, the fact that large numbers both pre and post independence were from the British Isles, and more importantly almost all the upper classes were English speaking meant it was never going to change. When business, govt, public announcements, and most media are in English it keeps it going.

Thus you would need sufficient concentration of speakers who would not learn or didnt know English to make another language viable even locally, of course urban neighborhoods worked for that, and still exist today with the various Chinatowns, or say Williamsburg in Brooklyn with its Hasdic Jewish community.

While there were places like this for German Americans, especially in the Midwest, and even survives in its own way with the Pennsylvania Dutch spoken by the Amish, the World Wars did much to force assimilation on many German Americans. Both outright efforts to minimize German culture, and simple social pressure took over. As an example of the effect this had, you even saw whole towns renamed to erase German influences, Berlin Michigan became Marne Michigan.