The reason is that I believe electronics and software will become ever more important. Traditionally the German manufacturers have outsourced anything to do with software and even most of the electronics. They have no internal knowledge about software what so ever - every bit of software know-how is at the suppliers. That's why I think that at least some of the suppliers will be better off in the future than the manufacturers themselves.
And it is also a question of human resources. In the past and for a long time software developers at German car manufacturers have been looked down on by all the "real engineers" who are in charge. As a software dev you have been much better off at a supplier. This is changing but in my opinion it gives the suppliers a good head start on capable staff for software projects.