Because everyone has the old notions of Vista from back when it was released. Its a much better OS now, especially now that hardware and software for it have matured. Windows 7 is even better imho, and despite a few quirks ( The nVidia drivers are not 100% stable even still, although that may be the folding im doing on the GPU's ) it works flawlessly.
Since I did a bit of cleanup and maintenance on all the pc's here, I took a survey of all the OS's we have. We have a FBSD 6.2 NAS, we have a FBSD 7.1 compile box that my dad uses exclusively. We have an old win 98 monster ( our first pc here in the US, it used to run 95 but we changed that). Couple sets of Windows 2000, couple sets of XP, a Vista laptop ( mine ), a Windows 7 desktop ( mine ), Ubuntu 9.04 in a VM, Notfred's SMP Client in a VM, and many, many old HDD's with diff OS' on them. Only thing we dont have is a mac, although with my sister owning an iTouch and looking at macbooks all day, that may change too.