I can think of numerous reasons.
1st off, Microsoft and Sony both fund advertising for 3rd party games, I'm not sure if Nintendo does the same for it's 3rd party titles. Advertising is a pretty big factor in hype.
2nd, As someone mentioned above, great graphics definitely help the hype, humans are very visual creatures and if you can catch they're eye you stand a MUCH better chance of being noticed. Take Zack and Wiki for example. I've heard it was a very challenging game that was fairly deep, before finding this out, I thought it was a game geared for ages 6-13 by simply looking at the cover :/
3rd Someone mentioned Nintendo's advertising strength residing in the fact that you can show people in the living room having fun frantically swinging around and such, but where does that leave the non-multi player games? Before I ever saw what No more hero's was about, I'd seen a video of a guy playing it and nearly crapped myself from laughter, it looked HORRIBLE, he was kinda just waving at random intervals, and doing this really horrid looking spin thing, all I could think of was how in the world a title like this got the hype, I felt bad for Ninty fans that this was the single player epic game they'd been waiting for.