I imagine it has something to do with the fact that Windows XP is the most boring OS that Microsoft is currently shipping. It has been around forever, and it just isn't sexy.
Lucky some folks have gotten other versions of Windows running on their Intel Macs. The list includes Vista (natch), Windows 2003 (my personal favorite Windows variant), Windows Media Center Edition, and Windows Tablet Edition.
So, when is someone going to get Microsoft Bob working on a MacBook Pro? The first person to do it will get a $20 iTunes gift card from yours truly (of course I don't even know if this is possible, but that's why it is fun!).