Looking back at why the Wii U failed

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It failed because where the Wii removed barriers to entry for gaming, the Wii U added them.

The Wiimote made gaming simple, accessible, and fun. The Wii U Gamepad made it cumbersome and convoluted.

It was a gimmick nobody asked for, it added to the price of the system, and the fact only one was supported per console complicated multiplayer.