Yes, Ubuntu/Linux may seem better to you (and to me, if it would just work on my system...), but consider the following:
Most of your average joe computer users aren't even aware that there even is an alternative to Microsoft software
Even if they know there are alternatives, they feel more comfortable with Windows
Perhaps they can't be bothered to switch, perhaps their favourite programs don't work, or whatever, many people don't want to switch to linux
I agree that Vista is a resource hog, but it is not just "XP with a new shell"
It includes many new features, such as DX10/D3D10, security enhancements, usability enhancements, support for hybrid HDDs, ReadyBoost, and so on
Personally I don't plan on upgrading any time soon, if at all (although I may be forced to when Vista-only games come out), but I just thought I'd put a word in for Vista
I don't like Microsoft software either, I only use it when I am forced to