Halo 2 is the creme de la creme of console shooters. Released in late 2004, Halo 2 sold an obscene number of copies and single-handedly made the Xbox Live service relevant. Halo and Halo 2 made the Xbox, and paved the way for Xbox 360. So it's understandable that Microsoft would look to Halo 2 as a launch title for Windows Vista. But Halo 2 is nearly three years old, and Halo 3 is only a few months away from hitting the Xbox 360.
However, the biggest problem with Halo 2 being a Windows Vista exclusive is it doesn't warrant being a Vista-only game. Aside from two multiplayer maps and support for high-resolution and antialiased modes, Halo 2 on Vista is indistinguishable from the original. There's no DirectX 10.0 support, squelching the one possible justification.