i also blame it on the fact that nintendo couldn't bank on a gimmicky console to help them this time. i'd say the wii's gimmick is better than the wii u's gimmick.
everyone saw how bad the wii actually was after it was out for a couple years. tie that in with the high launch price (compared to wii) and no game included, it means that the casual gamers who paid $250 for a wii-sports package isn't going to be spending $350 this time around.
The Wii appealed widely to non-gamers as it got them off the couch. That was really it's one and only strong point, and the list of its best selling games reflects this. Hardcore Wii fans are definitely rare though. Once the initial novelty wares off, a lot of them end up collecting dust.
The Wii U and its tablet on the other hand kind of feel stale to me. If you're going to game on a tablet, why not just game on a tablet. It sort of loses that edge the Wii had by being too traditional. To me the Wii U is closer related to the DS. That's not to say it's a bad system or poorly executed. From what I've seen, it does look pretty decent. I just don't think it has the same mass appeal. When the Wii launched, it was over a year before I saw one just sitting on a shelf. Can't say the same for its successor, which says a lot. Another thing hurting the Wii U is it being too similar to the Xbox 360 and PS3 in graphics and gameplay, both of which are on their decline.
The other problem is games. The Wii U only has one what I would call AAA title listed as coming out this year: Pikmin 2. So far it's the only upcoming Wii U game generating buzz on IGN. This seems to be a reoccurring trend for new hardware. Studios are very slow to get on board. I think this is mainly because they've gotten comfortable developing for current systems, which have finally become profitable. Nobody can be bothered putting in the effort making games for new, unproven systems. The hardware isn't selling because there are no games, and there are no games because the hardware isn't selling. It's a vicious cycle.
On that note, this is why I think Sony and Microsoft have chosen to use PC based hardware for the Orbis and Durango. It's already an established platform and porting games over should in theory be straight forward.