I think it's because Castrol has been a high performance specialty oil company from the Days of the British Empire. I used nothing but Castrol, in my motorcycles. It was Great Oil!
The German enthusiasts, car and bike, used it also. Today they are supplying BMW from production to formula 1 race cars. Who else has Autobahns?
Mobil is a big player too, Castrol is a Euro Corp. which may be a factor to the Germans.
Newb question: I see people talking about GC, German Castrol, right? Is there a difference between GC and what is sold in the US? I've used GTX for years, but was not aware that there were different "types" of Castrol oil available from different countries? What's the skinny on that?