My belief is that they didn't make enough motion control games.  The last really significant one was Wii Sports Resort.  This was released in 2009.  After that, their core fan base of motion control gamers didn't get a whole lot.  I think a lot of the games that were made to appeal to N64/Gamecube gamers were a mistake, e.g. Mario Galaxy 2 and Metroid: Other M.  Instead they should have doubled down on more motion control games.  They didn't make Wii U games to appeal to the Wii Sports crowd either which is the biggest reason why the Wii U flopped.