Motion and fitness where just the gateway mechanism. It seems Wii was based on getting people who didn't usually play games to play games - whatever kinds of games those might be.
There were a lot of theories about why lots of people exposed to computers, technology, and games, didn't play games. Were games too intimidating? Had interfaces gotten too complex? Did they take too long to play a session? Did the themes and stylization turn off the mainstream person? Were controllers too complex or uncomfortable for the uninitiated?
The Wii was very inviting and had an obviousness about it. The wiimote was designed to mimic both a TV remote, and a NES controller. Two super familiar items that pop-culture is familiar with and nobody is afraid of. Wii Sports was essentially what later smartphone games and tablet games would be - pick up and play, easy to understand, often with a social aspect, and colorful simple visuals with a clear interface. Very self-explanatory in terms of how to play it.
In a sense, it could also have been a content problem. Mobile games, social games, tablet games, exploded overnight and offered the mainstream plenty of friendly games that were already on a device many people had or desired for multiple uses. (Being cheap didn't hurt, but was maybe just a bonus.) One mistake Nintendo made with Wii IMO is that they stopped releasing a steady stream of those "blue ocean" games. After Wii Sports Resort, things just died off. Wii Party was an anemic, un-exciting final entry in the "Wii activity" series. In this sense, Nintendo's inadequate digital ecosystem may have helped do Wii in. They couldn't compete with the multiplying library of accessible blue ocean games showing up in the new mobile market. They didn't have a good delivery system for them, even if the games had gotten made.
To be fair, the DS also benefited from discovering some of this new market as well, thanks to its games such as Brain Training. It got a big boost. However, the DS still had one leg in the former handheld gaming market, a market that didn't completely erode with the rise of smartphone mobile gaming - at least in Japan. So the successor to the DS is not flopping once it got the software out there.
More general remarks -
IMO, Nintendo may have seen themselves as stuck with how to follow up the Wii. Wading into the red ocean of high dollar enthusiast game consoles and trying to find parity with Sony and MS may have been unappealing due to how much it would cost with so much danger of ending up with Gamecube II and nothing to show for it. On the other hand, following the Wii up involved trying to see if there was a way to appeal to people who might still be attracted to a revitalized Wii concept. There's a pretty obvious reason why Wii U didn't abandon motion control, the wiimote, and even accessories like the balance board. Nintendo brought those things forward with them to the next generation to continue working with them.
So in this sense Nintendo did exactly what was stated at the original unveiling of Wii U: tried to make a platform that was situated halfway between the "blue ocean" customer and the so-called core or "hardcore" game player. In terms of abstract, noble theory, perhaps it is not a bad idea, especially if you truly believe you cannot survive by copying what someone else is doing (like Microsoft, or Sony, or abandoning your own mobile platform and just going iOS).
In practice, as things stand it may have been a failed bet. Or perhaps they can squeak out something as happened with the 3DS.
The one thing that I think is silly though, and seriously projecting, is the common talk that goes "Well good! This horrible failure will break Nintendo of their arrogance!"
I honestly don't think it's arrogance. People project this onto Nintendo because they feel offended that someone makes a gaming product that doesn't measure up to their ideals and desires. Weirdly enough, this disappointment morphs into anger and the assumption that someone is trying to offend them. I rather think Nintendo's behavior is the result of the problem they've had for 15+ years finally coming to a head, as they try to figure out how to survive in some form that doesn't involve abandoning much of what they do.
Ironically enough, if you want to call them arrogant, I think you must go back in time to the dawn of the N64. That is when Nintendo made a huge mistake, very possibly out of some degree of pure arrogance or at least unwise confidence, and the N64 generation is when Nintendo truly got humbled. Maybe a lot of folks don't see this because they, in a personal sense, have beloved childhood memories of stuff like the N64 and the Gamecube. So history is framed to make those the golden years when Nintendo was right with the world. It wasn't. For people like me, already adults coming out of the SNES era, the N64 was seen as very problematic and got pretty bad by the end of its generation.
