Quote from Ryude
6 skills means I can use one hand to play my character, since I have the Diablo 3 mouse. It has 2 buttons on each side, so I never have to use the keyboard if I don't want to. If they added another skill slot, I'd be forced to use the keyboard. I'm not saying that is a bad thing necessarily, but it is something to consider.
6 Skills means = Keep synergy for console play, they won't go over 6 now