In addition to everything else, including Rol's shpiel, note that Nintendo themselves didn't seem to know what to do with the GamePad.
Look at the early Wii lineup. Wii Sports, Mario Party, WarioWare, Metroid Prime 3, and even Twilight Princess were either dependent or heavy influenced by motion controls. And although some games didn't use them as much, motion controls had a strong connection to many Nintendo titles up through the Wii's death. The motion controls, whether good or bad, had a bunch of games to justify their existence.
Now look at the early Wii U library. Besides Nintendo Land, the major releases barely touched the GamePad. NSMBU, Pikmin 3, 3D World, Tropical Freeze, Smash 4, Mario Kart... all of these could have worked about as well with either traditional controls or the Wiimote. Heck, I remember that at previews for Pikmin, people were encouraged to use the Wiimote/Nunchuck setup.
Comsidering how the GamePad's best justification, Mario Maker, cake out years after launch, I have trouble believing that Nintendo's devs were really enthusiastic about the GamePad.