Wii U had multiple issues but I believe software output along with the individual releases is one of the main problems. Let's take a look at Nintendo published titles during the Wii's first 18 months.
Wii Sports-November 19, 2006
Excite Truck-November 19, 2006
The Legend of Zelda: Twilight Princess-November 19, 2006
WarioWare: Smooth Moves-January 15, 2007
Wii Play-February 12, 2007
Super Paper Mario-April 9, 2007
Mario Party 8-May 29, 2007
Big Brain Academy: Wii Degree-June 11, 2007
Pokémon Battle Revolution-June 25, 2007
Mario Strikers Charged-July 30, 2007
Metroid Prime 3: Corruption-August 27, 2007
Donkey Kong Barrel Blast-October 8, 2007
Battalion Wars 2-October 29, 2007
Fire Emblem: Radiant Dawn-November 11, 2007
Super Mario Galaxy-November 12, 2007
Link's Crossbow Training-November 19, 2007
Endless Ocean-January 21, 2008
Super Smash Bros. Brawl-March 9, 2008
Mario Kart Wii-April 27, 2008
Wii Fit-May 21, 2008
U can see that there was never a drought between releases and their was a large amount of variety. U had things like Zelda, Metroid, Fire Emblem for the really hardcore Nintendo fans, the Wii series, Brain Academy, WarioWare for the more casual gamer, the big casual/core Nintendo titles like Mario platformer, Smash Bros and Mario Kart and some other titles that fall between those categories. Variety+consistency, something Wii U never had.
When the herd loses its way, the shepard must kill the bull that leads them astray.
