Thing is, Windows RT and Windows 8 are different beasts. Windows RT offers the advantages of a mobile OS, but only supports a subset of the applications available for Windows. Windows 8 supports most of the applications developed for Windows over the years, but is a clumsy mobile OS.
On top of that, I suspect that vendors and consumers are taking advantage of this shift to mobile devices. I suspect that a lot of people want to see the cruft that has accumulated in desktop operating systems over the years to disappear.