Because Windows 8 ships with Hyper-V 3.0, which relies on support in the OS for second-level address translation (SLAT). Without this, graphical performance inside the virtual machine is really hampered, and virtual machines require significantly more memory to run since they can't cooperate with the host operating system to dynamically allocate and deallocate memory as needed by the VM.
Keep in mind that a Windows Phone 8 device is going to have something like a dual-core 1.5ghz ARM processor. That's ARM, not Intel x64 like the host OS. The virtual machine environment has to be as fast as possible to deliver the kind of perf needed to consistently run at full speed. This requires cooperation from the host operating system. Windows 8 was designed for this. Windows 7 is not.