Because C++ is faster and can be compiled to machine code and, at least for now, you will need a 3rd party solution like Xamarin in order to compile and run C# on other platforms.
They'd be competing with Xamarin if they did that.
I think this is Xamarin, it's mentioned once on the page:
> ...or use these C++ libraries to build complete Xamarin Android Native or Java Native Interface applications.
It's a little unclear though
It is no Xamarin here! What they say is that you can use these C++ libraries from Xamarin or directly from Java (with JNI).