I will just say this: "cartoon" graphics aren't as big an issue as people make them out to be in terms of marketing, so long as they aren't in the area of WW in terms of "super exaggerated." I mean, look at Walking Dead episodic games or Borderlands; both are "toon" like, they just market themselves as badass instead of adorable. It's all down to the spin; you could sell boxed rocks with good enough marketing.
Also, the new Zelda is actually a graphic I could see people liking; it is kin to No Man's Sky in many ways (the colors, vibrancy, and some of the shading techniques) and people have praised that. They just need to keep their message focused on the action, open world, story, etc without dwelling on "cuteness" factors or anything.
But back on topic, the reason is fairly simple. The tech demos done on the GC and Wii U are just that: tech demos. What you are seeing is max capabilities in an isolated room with controlled events. A game running smoothly at that level (with the 1080p and high framerate) would have to be similarly constrained in terms of view distance and open spaces. The graphic in the full games are those that they feel allow them to do what they want while maximizing visual appeal and creating the flavor they want. I frankly like both visuals and would love to see both used this gen (with the second Zelda maybe being a cross gen title for the next Nintendo console in 2017 or 2018).







