Comment Not just that (Score 4, Insightful) 127
But both are gimmick based. The Wii's gimmick was the motion controller. That interested a lot of people, they thought it looked really neat and wanted to try it. Of course you discovered that it wasn't quite as cool as it first seemed, and many games really didn't play that well with it, but it drove console sales pretty well. People liked the gimmick and wanted in, so that sold a lot of consoles at least initially.
However gimmicks are fickle things, and there's no guarantee of what people will be interested in. The Wii U's gimmick is a tablet. That just isn't working out. People aren't that interested. Makes sense, since most people who wish to have a tablet already have one in another form and a game console with a tablet isn't all that interesting.
It also made the price less attractive. That tablet isn't trivial cost wise, so Nintendo couldn't be quite as low priced. That was something else that helped the Wii. It was low cost enough compared to the other two to be interesting to people who didn't want to spend as much, as well as people to get it as an "and a" console in addition to whatever other one they liked. The Wii U wasn't quite as price competitive and so didn't see as much of that.
Basically Nintendo got lucky with the Wii. It was the right gimmick at the right time to catch on and sell a ton. This time around, they missed big time.