The real reason third party developers don't usually support Nintendo hardware has nothing to do with power. It's because Nintendo's first party IP absolutely dominates their own console's marketplace. The PS3's best-selling third party game was the system's third bestselling title. The 360's was #2.
The Wii? Ubisoft barely managed to squeak one into the top ten.
There's just not enough of a market for third party games on a Nintendo platform to support developing a AAA game. AA is about as good as a third party dev can get, so they have to make games which will attract attention on a reasonably low budget. In other words, focus on gameplay or story, not on graphics.
And again, that's not the hardware's "weakness" making that necessary: it's the competition from Nintendo's first party IP.
Now compare to this to Sony and Microsoft: a powerful third party marketplace, and you market your games with trailers featuring incredible graphics. Nintendo systems go against every instinct a third party developer has.







