I suspect that it's not allowed because it would be an upgrade accross two versions: XP -> Vista -> Win7. (It's like trying to upgrade directly from version 1 to version 3 of some software product.) It's common for software companies to not provide for upgrades across two versions. XP -> Vista is OK. Vista -> Win7 is OK. But XP -> Win7 is not OK.
There are third party options available for upgrading.
See post #13.