Good question. Here's what I think:
Nintendo tends to make one installment of a franchise per system. Sometimes two like in the case of Mario Galaxy, but as a general rule they like to stick to 1. This makes sense in a lot of ways -- Nintendo would much prefer to have an evergreen title that keeps selling years after release than spend resources making similar games year after year. And it helps avoid diluting the brand.
The question for Nintendo becomes: would users really want to buy another copy of Mario Kart for the Wii? Can they do anything to make the new Mario Kart so different that it demands a new purchase?
The answer (to Nintendo) is no. Sure, they could make it use Wii Motion+, but I think it's obvious that Wii Motion+ is not enough to offer an appreciably new experience for Mario Kart. But the Wii U is dramatically different and allows them to offer a brand new experience to the consumer. And not only that, it offers that opportunity for all of Nintendo's core franchises, and also allows them space to create new ones (like Wii _____ on the Wii).
Nintendo is really in the business of making new experiences that people will love, and then packaging them in familiar brands so that people will give them a try. A new platform allows that a lot more than retaining the current one.
I think this is an extremely accurate distillation of Nintendo's thinking on this issue. It's also something I used to believe, but my mind is starting to change. Specifically, I no longer agree with the idea that sequels need to offer a new dimension to the gameplay experience to justify their experience, or to ward off staleness. Look at the yearly or semi-yearly iterations of the COD, Uncharted, and Assassin's Creed franchises. Yes, each game is bigger than the last, expanding the game universe. But as far as I know, the core fundamentals remain in tact. More importantly, look at some of the most important games Nintendo has made:
Super Mario Bros.
Super Mario Bros 3
Super Mario World
New Super Mario Bros.
New Super Mario Bros. Wii
Super Mario Kart
Mario Kart DS
Mario Kart Wii
Mario Kart 7
The Legend of Zelda
A Link to the Past
Metroid
Metriod II
Super Metroid
Donkey Kong Country
Donkey Kong Country 2
Donkey Kong Country 3
Donkey Kong Country Returns
Super Smash Bros.
Super Smash Bros Melee
Super Smash Bros. Brawl
All of these games maintain the same gameplay framework of the original games and yet, with the exception of Metroid and Zelda, all of these franchises have modern installments that match or exceed the sales of earlier iterations. I skipped games that changed the core gameplay, but it should be noted that many (but not all) of those did not sell as well as the games listed. I used to believe that people would get bored of a franchise if the gameplay wasn't radically shaken up, which would cause the franchise to head into decline. But sales data seems to fly in the face of that theory. Over the course of the NES, SNES, DS, and Wii generations, customers seemed perfectly willing to buy multiple iterations of games with the same core mechanics. In fact, one could argue that changing the core mechanics of a franchise is more likely to cause sales decline, but that's a different subject.
My current feeling is that
once a franchise has been established people want consistency in the gameplay experience. Why wouldn't they? If they wanted new gameplay, they'd buy a game from a different franchise. When people pick up a Super Mario Bros. game, they expect tight 2D platforming. When they pick up Mario Kart, they want an arcade kart racer. That's not to say that people want boring retreads. Expanding the game world and creating new scenarios seems to be an important part of driving sales for sequels. I also acknowledge that saturation is a factor, as Guitar Hero clearly showed. But I imagine the amount of sequels per gen that the market can bear varies based on game type and popularity.
So I do think that Nintendo can get away with releasing two Super Mario Bros., 2 DKCs, 2 Mario Karts, and maybe one or two entries in the Metroid and Zelda franchises per generation. The NES and SNES generation seem to support this, as do the sales of NSMB DS/NSMB Wii, and Mario Kart DS/Mario Kart Wii, which came out on different platforms, but within a short span of time.
This isn't to say that customers don't crave new gameplay mechanics. But I think new gameplay mechanics belong in new franchises. Changing the mechanics mid-franchise would be like introducing horror elements to the Star Wars franchise or a murder mystery into LotR. Every popular franchise should maintain the basic gameplay that made it popular (barring minor improvements) because that's what the franchise is known for. Sticking to the core concept allows the customer to be more confident in buying the latest installment of a popular series because they know it will perform the job they are trying to get done.