Because as soon as Microsoft blocks their software, they will update their software to change it's signature and make Vista run again.
The question becomes one of their ability to keep ahead of Microsoft, and how willing Microsoft is to risk breaking systems to play with low level hardware and BIOS calls. Those things are a very black magic and the whole point of large parts of Vista* is to avoid them. I don't know enough about virtualization on PC to make a guess who is going to win that race.
Jay
*And Linux, and every OS since Windows 3.1 for that matter.