Back when the Wii U launched it was a multitude of things that made the console fail.
First was messaging, many people (including myself) were confused at least initially what the Wii U actually was. Was it a controller that hooked up to the existing Wii or a new console entirely? Not until you looked at the background in the trailer and seen a differently designed console does it become clear. They focused to much on the controller/tablet itself rather than the console that left confusion. Which caused the casual market to believe it was nothing more than an overpriced peripheral for the existing Wii.
Nintendo had third party support initially (Batman, Watchdogs, ZombieU, etc.) but it wasn't able to be sustained due to lack of sales and interest as a result that most of the third party games were on the market for a while already. Another issue was that the gamepad itself was a 'one of' and wasn't easy at all to buy another one. It also lacked big hitting games from Nintendo at launch unless you consider NSMB a 'heavy-hitter'. The WiiU also had an issue where it was trying to cater to the casual crowd due to the Wii success. Which ended up not working like the video chat app, etc.
The big differences between the Switch's success so far and the WiiU is that of messaging (you know without a word stated what the initial trailer wanted to detail its a console that can be brought wherever as well as play at home on a tv), games (Nintendo brought out a heavy hitter in the name of Zelda at launch), pricing (its cheaper than most Mobile tablet devices out there), and design.







