Do you honestly think 90% of Wii U owner are normal customers? Wii U is probably 70% owned by Nintendo loyalists and 30% kids that who probably feel tortured by their parents with the Wii U, hence the shit numbers it has sold. Most normal customers didn't see any value in Wii U and jumped on PS4 this gen.
70% nintendo loyalists is ridiculous. Here are some game sales (from this site) of wii u software. Games every Nintendo loyalists would most likely own.
Donkey Kong Tropical Freeze: 1,4 Mio
Pikmen 3: 1,06 Mio
Xenoblade X: 0,8 Mio
Zelda: Twilight Princess HD: 0,75 Mio
Here you have your "loyalist" Nintendo fans on Wii U. Around 1-1,5 Mio people (10%).
Games like Mario, mario kart and splatoon are mainstream games, that got those high sale numbers.