Thanks for your approaches to explain Horst's success, Hamish and Scrump. It's true that there was a more greater variety of music than in follow-up decades.
But nevertheless, I'm going on to believe in heavy promotion organized by the record company (Polydor ?) or another event which led to this success because I cannot remember chart entries by german artists with comparable success. Even Bert Kämpfert didn't manage to enter the top ten with his original orchestra. And he was much more "better" than one hit wonder Jankowski.
On the other hand it well can be that there was a subliminal demand for this tune in connection with "Black Forest = Schwarzwald" which possibly was/is regarded as typical for Germany like Heidelberg, Loreley, Sauerkraut, Edelweiss (by Vince Hill ?) and other things.

Manfred