It's the games, not the graphics. You're going for the wrong conclusions here. The Wii's game lineup was affected by the combination of the Gamecube doing badly and alienating third parties, the 360 having made many devs target their current games at HD specs and initial third party efforts being about, well, party games. That led to a massive misallocation of resources with minigames and other cheap crap being thrown on the Wii. Also the HD games more consistently sold based on their production values* which are easy to control compared to harder to grasp factors like what makes a game good in the eyes of the "casual" gamer.
The Vita is flopping precisely because of its game lineup which seems to be driven by publishers not expecting many sales from the platform and thus putting less effort and worse teams on it along with targeting the games to be non-canonical spin-offs (i.e. second class games). Well, okay, that seems to be a general issue for handhelds ranging back to the days of the original Game Boy (think Super Mario Land). When a home console series receives a portable version that often ends up as basically a licensed game except it's not licensing a book or a movie but another game and the same rules apply (for an example just look at the new Call of Duty on the Vita).
Unlike the Vita the 3DS's primary catalog is not based on these second class spin-offs. Nintendo itself has learned to make the handheld games on par with the console ones (except NSMB2) by treating them as separate games instead of tie-ins (e.g. Mario Kart) or just going for completely different games (e.g. Kid Icarus), third parties have shifted to primarily making dedicated DS/3DS games rather than those spin-offs. The Vita has a few of its own games and usually these are considered the best for the platform but most of its games are these second tier spin-offs. Plus there are some additional features needed for a handheld game that a home game doesn't need as badly such as easy quitting/continuing and being engaging even in short bursts.
Also fun is that people somehow expect a handheld to be cheaper than a home console. Think about it, in addition to everything a home console has the handheld needs to be small (affects heat handling, the components must be more efficient), have a sizeable battery and includes a screen (which can get pretty expensive) and speakers. The only thing a handheld (better) doesn't have is a drive for optical discs. With PCs it's entirely normal that a laptop costs waaaaay more than a comparable home PC and often still includes a lot of drawbacks (unable to upgrade, OS bloated with what amounts to advertising, etc).
I'm not talking about console sales. I'm taking about how popular the consoles are among gamers and developers. The Xbox 360 and PS3 get all the big games, and are more popular than the Wii among (core) gamers.
The "Developers" part of this doesn't quite hold true either. The PS2 was quite a bit below the power of the XBox and the GameCube - yet developers flooded it with a lot of top notch exclusives and titles.
That would be publishers, developers hated working with the PS2's contrived hardware and its many limitations. They rejoiced when they got a new hardware generation that freed them from those BS constraints. However I do not believe that rejoicing affected the platform decisions of their companies, those are 100% driven by economic plans.
*= Of course not all the time. I think the expectation that production values = sales is what made EA cry foul over having MoHWf slammed by critics, in their economic models a game with that much money spent on it has to receive higher scores.