Because the Wii U was a stupid idea implemented even worse. The tablet was effectively tethered to the system and could barely let you move outside visual range of your TV. It made it completely redundant as a concept. The low resolution of the tablet's screen also did not help at all, as well as the washed out colours. It wasn't the power that did this, and the naming fiasco in my opinion is exaggerated. Switch has many other new issues, such as Nintendo seriously dropping the ball for the first time on quality of manufacturing standards (all my Nintendo controllers, bar my first N64 controller still have no drift -- but on Switch I have already witnessed drift on more than 4 out of 8 joycons that I have owned; my SNES and NES controllers that are 25 and 32 years old still work and function perfectly; my first Game Boy from 1989 still works as the first day, if only for slightly washed out screen and muffled sound). Switch is, also, ergonomically inferior (to an already ergonomically horrible Wii U). Ergonomics have generally gone down in quality in recent gens with Nintendo, after the incredible work done during the Gamecube era.
But at least the Switch is not a stupid idea. It is a brilliant idea. It is the best handheld of all time, and allowing you to play it on a TV screen is a fantastic concept that evidently works. It is Wii U in reverse.