Maybe since Penske owns Detroit Diesel as well, they thought that this was enough and their 50% stake in VM was good enough?
The real story here is that Russian companies are flush with cash to make investments in Europe and elsewhere. They bought the old Sebring/Stratus from Chrysler, are buildilng alliances with Western companies like Renault and Ford, and are looking to consolidate their own auto industry for a few "national champions" to take on the rest of the world.
The next couple of years will be interesting as India, China, and Russia push into more world markets. Don't be surprised if the company that ends up with Hummer is Russia.