Let's make a comparison of what was available on the Gamecube and Wii U systems about one year after launch:
Gamecube
- Super Mario Sunshine (Full fledged 3D Mario title. Brand new gameplay concept.)
- The Legend of Zelda: The Wind Waker (Brand new Zelda adventure.)
- Luigi's Mansion (Brand new series.)
- Animal Crossing (Brand new series.)
- Metroid Prime (Brand new take on the Metroid series.)
- Star Fox Adventures (Brand new take on the Star Fox series.)
- Mario Party 4 (Old concept. But great system-seller nonetheless.)
- Super Smash Bros. Melee (Best selling Gamecube game. Widely praised for its gameplay.)
Wii U
- Nintendo Land (Brand new series with several new gameplay aspects.)
- New Super Mario Bros. U (Previous entries were released for the DS, Wii and 3DS systems.)
- Super Mario 3D World (One previous entry found on the 3DS system.)
- The Legend of Zelda: The Wind Waker HD (Port with some improvements.)
- ZombieU (New concept. Low mass appeal.)
- Pikmin 3 (Sequel to a declining sub 2 million franchise.)
- Lego City Undercover (New take on an existing series. Weak as a system seller.)
- Monster Hunter Tri (Widely established series... In Japan.)
Now imagine that you, or any consumer for that matter, never owned any of the two consoles and ask yourselves which one looks more appealing. Also keeping in mind that the Gamecube is cheaper than the Wii U.
...
Point being that while some people are surprised that the Wii U is tracking below GC, I'm not.