The sales story
Wii U's launch never produced the casual-audience surge that the Wii had built across the previous generation. The hardware identity was unclear to mainstream buyers: a tablet-style controller paired with a home console that looked, from a distance, like an accessory rather than a successor. Marketing struggled to explain what the platform was meant to replace.
What kept the platform credible was the software it eventually shipped. Mario Kart 8, Super Mario 3D World, and Super Smash Bros. for Wii U showed that the first-party output was as strong as ever. But Nintendo could not turn that catalog into the broad hardware spike that the Wii had produced, and the third-party pipeline thinned out faster than on rival platforms.