Because C/C++ is used for big real apps. Almost none of the programs you use on a day-to-day basis are written in anything other than C/C++:

The NT Kernel: Written in C
All of Office (apart from Exchange): Written in C++
The Explorer shell: Written in C++
All of your device drivers: Written in C or C++
Wireshark: Written in C++
Visual Studio: Written in C++ (some of the GUI is managed C++)
Open Office: Written in C++
Any popular game that you care to mention: Written in C++
Skype: Written in C++

You get the drift. C# is great for mini-apps and apps for internal use in your company, sure, but when it comes to apps being sold for real money, C/C++ is where the industry is, and it's hardly surprising that Microsoft takes steps to make them happy.