I voted for using XP because of low system ressources.
I don't need Windows anymore for my daily work. I just use Windows 7 to play a Windows only game, that doesn't run with Wine every other week.
But somethimes I do need Windows to do a little work, mostly when I get Photoshop files that don't work with Gimp. I compared Windows XP and Windows 7 in VirtualBox and Windows XP needs far less ressources and Windows 7 has not a single feature that I improves my work experience! Everything runs slower compared to XP!
So that might be the reason why most people still use XP: They don't need the features of Windows 7 and their computers might be old but working with XP! For Windows 7 they would have to upgrade their hardware but they don't want that!
In fact the number of 66% might be to low, I know many people who buy a computer with Windows 7 and downgrade to XP!