if people wanted to play games on a tablet they'd probably do so and on one that could travel around with them.
Touch-screen tablets are good for positional input. But some game genres are more suited toward directional input than positional input, especially things like platformers and fighting games. Trying to do directional input with a touch screen is painful, as shown in the demo of Pixeline and the Jungle Treasure. The advantage of Wii U GamePad over a tablet is that it has both traditional gaming controls (a directional control and discrete trigger buttons) and a touch screen. Only a few uncommon mobile devices, such as Sony's Xperia Play, Archos GamePad, and JXD gaming tablets, have both.