If you see the chart below, the question that rise is “why Windows Vista Basic/Home Premium users can upgrade to Windows 7 Ultimate but not Windows 7 Professional?” others perhaps will ask “why Windows XP can’t be upgraded to Windows 7?” but in the case of Windows XP, I think the answer is because XP and 7 have a very big gap of architecture hence you can’t upgrade it.
Windows XP users have to go the longer path if they wanted to upgrade to Windows 7 instead of doing fresh install. First you have to upgrade to Vista and then you can upgrade to Windows 7. Based on XP to Vista upgrade chart this can be done, see the upgrade chart below.
So For example you want to upgrade from Windows XP Professional to Windows 7 Professional, then you have to do:
1. Upgrade Windows XP to Vista Business
2. Upgrade Vista Business to Windows 7 Professional
Microsoft has an offering for users that upgraded to Windows Vista, they will eligible for Windows 7 upgrade when Windows 7 released on October 22nd, for more info see here.