I think that WWI and WWII are to blame for the disappearance of German being spoken in the United States more than anything it does not help that it generally speaking is expected in America that one assimilates which typically means the loss of ones "mother tongue" usually after only a few generations.
At one time German was the second most common language in the US.Which again is why I say that WWI and WWII are to blame for the disappearance.
For example some of my family originates from Germany from what I understand pre WWI it was very common to speak German publicly around other German speakers in German neighborhoods.WWI made this a bad idea so after that it would only be spoken at home and by WWII and even before the cultural model displayed by Germany was not exactly popular with all German-Americans so many choose not to speak German around strangers so as not to appear pro Nazi.
Many second and even first generation German Americans fought in WWI and this event would also change your identity and make you feel more strongly for your adopted nation.
If you look at most any ethnic group that has been in the US for a few generations the trend will be about the same.Very few Italian-Americans would be able to have a conversation with an Italian.Most Japanese Americans do not speak Japanese.Most people of Scandinavian decent can not speak Swedish,Norwegian or Danish.
Of course the United Sates is not unique in most nations your are expected to assimilate.
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