That's because having US citizens as customers requires them to file tax information for those customers with the US government, and the German banks don't want the overhead.
And that's because for some unknown reason the US is one of the few countries that tax based on citizenship, and not residence [1]. It's absurd. Even resigning your citizenship is taxed [2].
As someone who has paid a truckload of these taxes: I don't think it's that absurd. You're not double-taxed. And if you want to relinquish citizenship, you can.
[1] - https://en.wikipedia.org/wiki/International_taxation#Citizen...
[2] - https://nomadcapitalist.com/2018/06/16/tax-consequences-of-r...