The bigger question is the other way round - why do so many non-Nintendo games wither out after a few months?
I think it comes down to two things, competition and online play.
Big-budget genre games are in competition with each other for gamer attention. They have to get that attention through advertising, and given the competition you have to front-load the advertising dollars and push the game hard at release.
You then get diminishing returns from advertising spend further down the line.
The Nintendo titles have less competition (for example, there really isn't any serious competition to Wii Fit, there wasn't anything to match Pokemon, Endless Ocean is uniquely relaxing), so they need advertising for awareness, but the budget isn't pushed up by competition and they sell largely on word-of-mouth.
Online affects it because anyone who wants to play competitively online has to get in early or they will get continuously pummelled. Again this works to frontload sales.
Ok, three things, there is Nintendo's spectacularly good brand reputation. They don't do turkeys.