I would guess because none of the developers have a Windows Phone or they don't have any interest in the platform. And no one has joined the team to develop a port.
How can you develop applications in C++? It seems like it's still limited to a very small number of applications that Microsoft have blessed, or people who have jailbroken their phone, still..
Microsoft speakers confirmed that developers will be able to develop apps for the “next release” of Windows Phone using C++ native coding, although the speakers refused to confirm whether this “next release” refers to Tango, Apollo, or the next developer release of Visual Studio