Because the casual market collapsed due to smartphones and tablets and the Wii brand had become a toxic liability as a brand that was out of style for one.
A "Wii" system with the "Wii audience" gone to their far more fashionable/trendy iPads and iPhones was always going to crash and burn. If Switch was called "Wii 3" it would be having brand problems also.
The GameCube did not have Wii Fit or Wii Party U or Nintendo Land or NSMBU ... all central tenants of the Wii success or borne out of the Wii philosophy.
People got tired of mini-game compilations and 2D platformers and guess which two genres Nintendo bet the entire farm on because they did well on Wii for the Wii U's first year.
Terrible marketing aimed at kids and casuals primarily.
Awful controller that looked like a toy.
Bad post-launch lull.
No "epic" Nintendo IP early on in the life cycle.
Take your pick. They're all here.
