I already know the answer: MS screwed up. They decided to ignore the fact that lots of people still are on XP and do a one version upgrade instead of the industry standard 2 versions. It’s not complicated, MS has a long history of screwing things up, if we’re lucky the lack of XP upgrade will be the biggest screw up in 7.
It may be more basic than that. There’s more $ in buying a boxed operating system than in buying an ‘upgrade’. They felt ‘compelled’ to do something about the Vista issue. They didn’t feel compelled to do something about other legacy systems.