Well for one, the consumer was sold on an idea that didn't actually exist. Like most tech promises (looking at you hololens) the reality falls so short of the imagination it leaves a bad taste. Initially, it does really well, but the limitations start showing up rather quickly when a concept that the consumer falls in love with doesn't pan out.
Furthermore, the type of gamer wii and kinect targeted were lapsed gamers. People who wanted to relive their childhood. As we know, the games industry is an imperfect one, especially with concept tech like this. The downsides quickly outweigh the upsides. Time wasted playing games, cost of games and tech, lack of expected performance and growth.
The wiiU was a really good idea that was unfortunately crippled by the first party tablet. Some of the ideas were really cool and interesting, but the tablet kept the cost prohibitively high and was a bit cumbersome.
I know you're thinking hey these aren't major problems, these are just annoyances. But when you look at the bigger picture and realize that the competition for the casual audience isn't exactly limited to gaming, but mobile, tablet, pc, and actual games/hobbies.
The reason why systems like ps4 and xb1 can do well is because of the core and hardcore fans. They seek out titles that help them relax, or amaze them in unique ways. These people can be compared to something like movie buffs, or sports fans who have integrated their hobbies into their lives.
People who were out there buying wii and motion controls were in it for far less, and current mobile delivers on those needs without the price barrier.
If Nintendo wants to stay relevant, ie, increase their success, they need to both tap into the core market once again with strong hardware, as well as establish themselves in the mobile arena which they are already doing.
Maybe I'm rambling, so I'll try to say it succinctly:
Motion controls are only a small part of gaming and the transitioning to the casual market was very poorly handled.
