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.
I’m sure money plays into it. We know it’s not technical because 2008R2 (Windows 7 server) CAN upgrade from 2003 (XP server). They simply chose not to, for internal reason which I’m sure seem perfectly valid from Redmond, but from down here in Tucson it’s exactly the kind of short sightedness I get paid to rail against.