Because he's a German citizen, I think? But that's what his appeal is based on now. He was extradited from Portugal to Germany on drugs charges. Now they want to charge him with the rape of the elderly lady but he is appealing that because it didn't happen on German soil.
There has to be some way though, surely? It would be a hell of a failure and risk to public safety if he isn't able to be charged with anything else he's done outside of Germany!