Windows XP is mature Vista is not. People who surf the net and check email would have nothing to gain when moving to Vista. In fact, for many people the move will not fundamentally change how they use their computer.
Windows 7 is different. It actually feels faster. Things seem snappier, the taskbar is smarter, file sharing is simplified (complicated in some respects too). Microsoft gets the message now, people want a very performant OS (they have a long way to go before
many will be happy, operating system rot needs to be solved...) and a secure OS. Microsoft arguably has some pretty secure software now, and with Windows 7, they have something decently performant as well.