Simple. The exclusives weren't good enough. NSMBU was wanted but 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
SM3DW, Mario Kart 8, Splatoon, Mario Maker, Smash Bros 4, Zelda BotW...are all great and strong potential system seller games, but they all come out on Wii U when console was already dead and considered for fail, Wii U need some of those games on launch and in its first year, similar to Switch.