So it's widely known as a fact that many Nintendo games have ridiculous legs. But here's the question that no one really bother to ask...why is this so?

[Mnementh]

I disagree with the children-explanation. There are not that much children turning into a age they start playing videogames, that it explains this. I think it is something different.

It happens mostly to games with local multiplayer (like Mario Kart or Wii Sport Resort). I think it has to do with: people that are NOT owning the game yet play it over at friends and like it and buy it themself. It possibly even making them buying the console, and they already know which games to pick up together with the system. There are two things that are needed to fulfill this: can be played together with friends not owning the system (local multiplayer) and the game has to be of a great quality, otherwise it wouldn't work.

I agree to some point with the missing competition. Third parties can manage the same legs, for games that qualify in the same area. See Just Dance sales.