Windows isn't a mobile platform. They tried 2.5 times and have given up. In mobile it's iOS and Android, and neither Apple nor Google recognize C# as a first-class language. If you can write a compiler that emits code that LLVM can compile to ARM binary with Xcode or package your MSIL with a micro .NET VM on Android, then more power to you but Apple and Google aren't going to help you if you get stuck.
While still in Preview, Xamarin.Forms actually supports Linux with Gtk#, Windows with WPF and UWP and Mac with Cocoa (actually Cocoa with C# does work really well).