This is because Apple has already dropped support for older versions of Windows, and Boot Camp will support only Windows 7 and nothing else. For those who are using Macs at home with Windows XP or Vista installed on Boot Camp, you are able to stash away the Boot Camp partition, although you can’t make the upgrade to the latest version of Boot Camp.
For those who want to retain Windows XP or Windows Vista and yet want to purchase a new Mac or make that upgrade to OS X 10.7 Lion, I’m sorry that there is no compromise at the moment. All you need to do is just make sure you have a Windows 7 installer handy.