Windows on the desktop is a known and reliable brand, nobody can deny that. However, Windows on the tablet is an unkown today, and people seem comfortable and used to using a non-Windows OS in their tablets and phones, it's a radically different situation.

I think it's a mistake to believe that people will choose Windows 8 in their tablets and phones just because it's Windows. Otherwise it would have worked wonders with Windows CE and Windows Phone, and it didn't. There are other factors in play, and as I said in another answer, the world is not waiting for Microsoft to catch up.