Coming from an IGN interview with Masahiro Sakurai.

“You can think of situations where, if you implemented touch controls on Wii U, then the GamePad player would have advantages over other players. We want to balance evenly across Wii U and the 3DS versions, so there’s a balance issue there, which means you would have to avoid using touch screen controls.Back on Brawl, there was the possibility of incorporating motion control with the Wii Remote and Nunchuk controllers. Our priority has always been to allow players to use the controller they want to use. Having that balance amongst the different controllers is very important. We want to give each player the control experience they want while keeping gameplay balance.”