[bri3d]

Much the same as iOS exists as a fork of OSX and shares the kernel and a lot of userland frameworks, Windows Phone 8 and 10 were just forks of Windows and shared the kernel and even more of the userland. It's unlikely Microsoft would want to open-source that.

[vadimberman]

That sounds plausible but they do collaborate with the .NET open-sourcing efforts, that could be an opportunity for them to test new waters.