C++ has a much longer learning curve. C has only few constructs you need to be aware of and then you can start coding powerful software. In C++ you need to learn the C base, then the OO and generic programming, exception, etc. And after a time you may know most of the features and you porbably can use them, but you still don't know how the compiler will translate them, what implicit overhead they have or not. This takes much time and energy.
For a professional project this argument may not count, because you can employ people that already know C++ very well. But in Open Source Projects, where C is still widley used, the people pick the language they like and they are able to use. Consider that not every OS-programmer is a professional programmer.
