A lot of things went wrong, but one of the most important things was the godawful dry spells.
There were 8-9 months between the launches of NSMBU and Pikmin 3. There were 8 months between the release of Xenoblade Chronicles X and Tokyo Mirage Sessions. These are two massive dry spells bookending the Wii U's life cycle. From August 2013 to December 2015 Wii U had a good and steady release of games. Outside of that roughly 2 year period it was a barren desert.
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