Simple. The exclusives weren't good enough. NSMBU was wanted by no-one, same for SM3DW and the Zelda remakes. I bought one for Pikmin 3 and sold it soon after finishing it. There was nothing else there that I couldn't play on the 3DS. It felt like they were competing with themselves
I agree with you to an extent because while the overall top games the Wii U got is comparable to any other Nintendo platform, they all came too late. Nintendo placed all their hopes on Nintendo Land and NSMBU holding things down until reinforcements could arrive and both games failed. After that they had nothing to get them through the drought of 2013 and the console's reputation as a failure was cemented before Mario 3D World, MK8, Smash 4, Splatoon, Mario Maker, XCX and others could provide support.