I don't think the gamepad was ever a great idea. It was different, but never great.
The one area where it can shine is letting one person play differently than the others. Not sure how much they really got to use it - as it's an element of Fable Heros and upcoming game.
Plus the tablet was an expensive part of the hardware. And Microsoft showed off it's Xbox SmartGlass. While not really used (yet) in a similar way, it did defeat the momentum of the 'newness' of the WiiU Tablet.