Because Nintendo decided to abandon their most popular control scheme of all time and focus on the gamepad instead. Welcome to the real world: Motion controls were never "a fad", people loved the Wiimote. I've been asked so many times why the Wii U doesn't use motion controls anymore by friends who owned a Wii (and yes, I know you can connect a Wiimote).
Motion controls are as much "a fad" that people "tired from" as 2D Mario was.