I think the majority of German programs has neither a very international alumni base nor really a very international appeal in the first place. I believe most of the nationally high ranked programs are indeed well respected with Germany's top employers, but for a German who wants to get a top B-School experience, I think it is expected to to go to an English speaking country.
Now, if you really wanted to go a notch deeper, I think the history of German higher education plays a role here. German MBA programs are all still fairly new. Many of them were either Diplom courses before or were in its entirety created from scratch. As such they have not really had the proper amount of time needed to prove to the world that they can educate top leaders. Look at German engineers on the other hand, it doesnt even matter that you did not get your degree from the RWTHA, you are a German engineer - you are synonymous for quality.