Actually, the biggest things standing in the way of the original Wii when it came to actually selling games made by third parties was not the power of the device but rather a profound misunderstanding of who was buying the thing and what actually made the Wii interesting.
First, the most obvious, is that the Wii found tremendous success largely because it was successful in selling to people who otherwise were not terribly inclined to purchase games. Thus, the most successful games the console had were those that were barely games but rather an exploration of various trivial tasks the controller was good at. These include the various fitness programs, Just Dance, and the pack in sports titles. While there were certainly other successful runs of titles, they were exclusively Nintendo produced and every single one of them was part of a well established decades old franchise.
Other developers who tried to sell their own wildly successful franchises on the device ran into problems. The first simply that the console only had a fraction of the power of the other devices meaning any port attempt would not only take more effort, it would necessarily look different and, given the focus on "realism" for most of these titles, different generally equated to "worse". While there were attempts early on to put real effort into the process thanks to the tremendous install base the console enjoyed, they consistently ran afoul of both the problem above and problems further on.
Beyond the obvious power limitations, the Wii U had a secondary problem of a relatively dearth of storage space. Not only are the discs simply less capable of housing data, the console itself is lacking in storage capacity. This factor alone undermines a key strategy of various franchise holders as they attempt to continually monetize a particular game after launch through the consistent release of DLC.
And when it comes to DLC, you have to consider yet another problem: a complete lack of anything approaching a useful online service. While people can debate the relative merits of PSN and XBL, there is no argument that both services are reasonably functional, easy to use, and consistent across games. A number of other problems further exacerbates the issue until you arrive at the point where the best you can say about online functionality about the Wii was that it existed. This is rather damning when the biggest franchises of the era rely tremendously on a strong multiplayer component for their success. No matter how good your port of the single player portion of Call of Duty, few are going to buy the game for that reason alone.
Thus the problem with the Wii becomes relatively clear, a combination of lack of technical power and online functionality undermined early attempts to sell things on the console and given the sound business strategy of abandoning failure and reinforcing success this meant little was risked on future attempts on the device save by companies already pressed into corners by market realities (Sega for example). Profound misreads of the audience contributed to some of the problems which, when combined with the simple fact that after the first few attempts most games designed for a "core" audience did so with risk mitigation in mind meant that success was all but impossible to achieve. This is how the two best selling third party franchises on the console are some of the worst examples of games (A fitness game who's name escapes me and Just Dance).
This leads to the problem of the Wii U. While it is technically proficient and has improved online functionality, it is now fighting an uphill battle. Currently it faces a combined install base that is well in excess of 120 million devices with an install base of a few million units, a disparity all but ensuring it will not be the lead platform in the immediate future for any established third party franchises. That the device still has baffling technical limitations that could have been easily and cheaply corrected this time around ensures that much of the power it may have is going to be difficult to leverage. While the device has an interesting controller that manages to be both interesting and roughly comparable to the functionality of competing controllers offers an edge if and only if a developer uses it appropriately. The use of it to provide split screen functionality in multiplayer shooters for example is an interesting use of the technology for example but not one that will make the current iteration of Call of Duty sell tremendously on the device. Most of that franchise fan base is committed to playing the game on a different platform given that their friends play the game on a different platform.
And that is where the truly damning problem of the device lies. It isn't the technical limitations - those can be largely worked around. It isn't the fact that the device is only marginally more powerful than consoles released the better part of a decade ago. It's simply that the device exists in a middle space with little power to attract the casual or the core. The casual audience that made the Wii U sell so well and ensured the success of some of the biggest franchises on the device has little reason to make the switch. The rise of the facebook game and the smart phone app makes such a move pointless for an audience that never fully committed to the medium as a truly viable entertainment option. The core audience on the other hand is going to be just as difficult to sway given that they may have also spent the better part of a decade cultivating a network of people to play games with on the particular platform. Winning a few converts here and there is going to be hard enough. Much like MMO's in the face of WoW, the Wii U is going to face the problem of pulling a critical mass of the core audience to their device that ensures the long term survival and growth of any community.
Put all of that together in an era where people simply have less money to spend and you arrive at the stark reality. 350 bucks can be spent on quite a lot of stuff that isn't a Wii U and for many at this point there isn't a compelling argument that they should. In the face of an install base that is currently two orders of magnitude greater than your own, the slow growth plan simply isn't viable. And unless it makes enormous headway before the next generation of the 360 and PS3 roll around, the slim advantages the console currently possesses will have gone to waste.
In short, the problem isn't some trivial lack of power currently. The problem is simply that the previous market that made the Wii U a success as moved on and the device doesn't give a compelling reason for them to move back. And the market that long eluded the Wii only has a reason to move if their friends move. And without software to make them move, software that represents enormous risk in an era where every major developer is doing everything in their power to avoid risk is unlikely to something developers jump at. And all of this places the device in a Catch 22. The device has advantages that are only advantages if people capitalize upon them which they will only do if people buy them in tremendous number which they'd only do if the aforementioned software gets made. The Wii U doesn't have years for this to self-resolve.
