Because, Mac hardware runs Windows much much better than a PC does. I am not just sayng that because I am a Mac user. When I fire up my wife's Sony Vaio or my daughter's HP, it is a noticeable performance and stability difference than when I run W7 on my Mac. I have heard that same sentiment echoed from other Mac users. I
have never had a problem running both OS's under a separate platform using a bootcamp installation. Windows is great for games and I think that W7 Windows Media Center is the best free media center, so those are the reasons I tend to jump back and forth between the OS's.
Personally I like Windows 7, it's just to bad that Mac just doesn't build a pc for it....what a minute, Mac does, they are called Mac's.