Because motion controls is booty.

Motion controls were not accurate and often times didn't act how you would expect it to. It was also relatively uncomfortable because it required accuracy that the device simply didn't have along with awkward motions of your own arm. Motion controls didn't have universal use in all games like a standard controller or for us PC people, keyboard and mouse.

I personally did like the motion controls idea but only in a few games while in the majority of games it felt like it was in the way (taking feeling of control away) and some games resorted to gimmicks just to try and make use of it when it didn't need it. Something some people don't seem to realize will apply to VR, though I think VR will be more successful in sticking around as a bit more then a gimmick but it also is not some universal feature that works well with everything.

Motion controls are THE WORST. I perfer just a controller... Nintendo ruined Skyward Sword with motion controls. that game will forever be a bad game because of motion controls. 

Because people are lazy and always seek to minimize the effort needed to achieve a goal. Wasting body resources for motion control is simply not economic and if people want to relax they do not want all this motion bullshit. Motion controls ruined many games, since stupid shaking was required for almost everything.

Ultimately the problem with any new periphery is finding engaging, and deep experiences that are so compelling that people feel they must have them. This is why the Wii ended up petering off, and why the other motion control periphery have really struggled to gain any kind of traction. There were some fun games, but they weren't lasting, and they weren't anything that you'd say I've got to have the system and the MC peripherals in order to play this. VR setups will also need to prove themselves on this level, although, I think that will be an easier challenge to achieve than MC was.

Personally, I think Kinect really had some amazing potential. I own PS Move, and I had fun with it, but Kinect's ability to really track joint movement/placement was something I thought was intriguing. Specifically in the martial arts category, I thought it could be an exceptional tool to teach forms in, as it could track your movement much better than Move could. This was always something I'd hoped that Microsoft would try to achieve. Maybe someday in the future.

Lack of accuracy, lack of good implementation overall (we want to relax, not work out), and lack of game series that actually benefit from them. Most importantly, lack of financial benefits for companies to bother; Wii had it built-in, but the number of PS3 owners with Move is small in comparison, same with 360's Kinect, and porting such games between three different motion controllers requires more dev time than a typical game.

Motion control didn't work as promised initially but as time progressed, it got better for all three console makers. Right now, the WiiU, PS4, and Xbox One have MC that works almost as good as we hoped last gen would but hardly anyone cares to take advantage of it.

People are lazy and don't like to move.

Because it´s time to move on

Motion controls hit the mainstream so hard people don't even think about them any more. Think about how many devices have gyroscopes / accelerometers and how many games use them without you even thinking about it.

Think about all of the people who say motion controls were a fad and virtual reality is the true next big thing. Think about them and laugh. The joke is that gyroscopes & accelerometers, AKA motion controls, are the core technology which allows VR to work.

Motion controls can and do coexist with other control types. People just prefer to 'see no evil' and pretend it doesn't exist. They don't think of Star Power in Guitar Hero as something that is triggered with motion controls. They don't think of Journey as a motion-controlled game because it just maps camera controls to the gyro.