I think one major reason why XP succeeded was that it actually made a lot of difference. Compared to Windows 98: Way less crashing, way more stability, and a much better platform to make serious software on - thanks to a modern 32-bit kernel with proper segregation between userspace programs, which finally became widely available and tested.
What does Windows 8 bring over Windows 7 that justifies an upgrade? Small incremental updates don't count, Windows 7 has those too (over XP), yet it's still far from universally adopted after over 3 years.
(Now my lens may be colored with my view that convergence between laptops and tablets is a stupid idea, so I don't really consider that as an actual advantage of Windows 8.)