Because Vista has DX10, has a more secure/stable driver model, and is the only MS OS that is wholly tested with IPv6. As a bonus they are starting the transition away from using the registry.
1) the new driver model neccessitates rewrites of existing drivers, meaning you are using in essense, *BETA* drivers developed by developers who had to learn a new architecture. This will initially result in LESS STABILITY/MATURITY, so current Vista users are essentially guinea pigs.
2) IPV6 is IRRELEVENT first of all. Secondly, TCP stacks are notoriously difficult to get right, and MS through out a TCP stack that was mature with a long lineage and replaced it with one that was rewritten. It tooks decades for all the bugs in the BSD and NET TCP stacks to be found and removed, and I'm supposed to trust Vista's rewrite that has no public auditing of its source?
Vista has already crashed on me several times, in fact, it crashed in the SATA disk driver one time. There is no goddamn reason to use the "1.0" version of ANY Microsoft product. We all know that that the safest time to upgrade will be after about a year from now, when many of the initial bugs are fixed, just like with NT, 2k, and XP.
So I will repeat, there is little reason to go Vista given no DX10 games, unless you care only about the idiotic UI chrome redesign, the one where you the active and inactive windows look almost identical save for the 'X'
(ok, well, there is the UAC-super-annoying-cancel-or-allow-everything, providing a false sense of security, where you could fix 99% of your security problems by just not using Internet Explorer
)
This whole "Vista is must have, so therefore, unstable GPU drivers are a huge market killer" is one of the most ludicrous arguments I've ever seen. Vista today is by and large, only relevent for people buying new computers, and people with a deathwish who want to buy expensive($$$) 1.0-release MS software.