(without looking at details) The problem is that Windows and Linux expose hardware and drivers in different ways. You can shim things up to make the code work, but you end up with a driver that doesn't look like a Linux driver and doesn't work like a Linux driver and can't easily be maintained by people working in the Linux graphics drivers is going to be a problem.

If the driver doesn't really belong in the Linux kernel source for those reasons, it's better to keep it outside the kernel tree.