The reasons range from the look and feel of the desktop, to a program that they felt more comfortable with, and it's no longer available with the new Windows 7.
With that in mind, Microsoft has created the Windows XP Mode, for Windows 7. Basically, you run a virtual version of the XP OS, inside the Windows 7.
Designed primarily with small- and medium-sized businesses in mind, Windows XP Mode comes as a separate download and works only with Windows 7 Professional, Ultimate, and Enterprise. Windows XP Mode also requires virtualization software such as Windows Virtual PC. Both are available free on the Microsoft website.
And it's FREE! You get a fully working version of Windows XP Professional SP3, and can load any of your non-windows-7-compatible programs. The nice thing is that you can even open these Windows XP programs from within the Windows 7.
Though this would come to be unnecessary for most, this is great news for some.
Learn more, and download files needed "here".