Because of the (taking into account the extra stuff it's always doing) insanely huge waste / bloat of resource usage.
To those who say it's "justified" because of the flashy GUI, go install linux along with compiz fusion on a PIII 700 with a geforce 2 or something and watch it fly along doing a gui that can look and act exactly the same as vista with no problems at all.
http://en.wikipedia.org/wiki/Protected_Video_Path
http://en.wikipedia.org/wiki/High-bandwidth_Digital_Content_Protection
http://en.wikipedia.org/wiki/Image_Constraint_Token
http://en.wikipedia.org/wiki/Criticism_of_Windows_Vista
http://badvista.fsf.org/what-s-wrong-with-microsoft-windows-vista
http://it.slashdot.org/article.pl?sid=07/06/22/139247
As for gaming, has any company even dared to release a game for DX10 ONLY, effectively blocking all pre-vista OSes? Games that you can run on both of course run faster on XP (last wiki link above, read).
If a game came out now that I wanted to play and the developer was stupid enough to force vista, I'd get the console alternative (if any) or not play it. Yay for bioshock having dx9.
This is of course ignoring the fact that many supposed DX10 "only" games (including Halo 2 for PC) have been fixed by crack groups to run on XP / DX9 (http://thepiratebay.org/tor/3720411/HALO_2_XP_Patch____). /
http://thepiratebay.org/tor/3729197/[PC]_Halo_2_XP___VISTA_[RIP]_[dopeman]