In not sure how much this affects things but the handheld and console this generation are more similar than any generation before it.
Gameboy vs Super Nintendo-4 colors vs 32,000 colors
Gameboy Color vs Nintendo 64-2D vs 3D
Gameboy Advance vs Gamecube-2D vs 3D
Nintendo DS vs Wii-mostly 2D, touch controls vs 3D, motion controls
Nintendo 3DS vs Wii U-3D vs 3D, both have motion/touch controls
Not only that but many of the games are similar
Super Mario 3D Land vs Super Mario 3D World
Zelda:Ocarina of Time 3D vs Zelda:Wind Waker HD
DKC Returns 3D vs DKC Tropical Freeze
Mario Kart 7 vs Mario Kart 8
Super Smash Bros 3DS vs Super Smash Bros Wii U
Then u also have to consider 3DS also has Pokemon X/Y, Animal Crossing, Fire Emblem and many others, costs half the price and a family is more likely to own multiple handhelds compared to consoles. Obviously 3DS isnt the only problem or the biggest problem but its very possible it contributes to sluggish Wii U sales