Other then the softwares appeal, a big reason is Nintendo's marketing. I've seen TV adds for stuff like Animal Crossing DS and New Super Mario Bros 3 years after they were released, thats the key thing. I'm not sure if third parties could attempt something like this though, as that sort of marketing budget is hard to justify without trying to sell consoles
Advertising an older game is more cost effective for Nintendo than it is for a 3rd party publisher. Nintendo is the console maker, publisher and developer of their game so they get to keep the approx. $30 a publisher would take in. Plus the commercial also advertises the Wii or DS. A 3rd party publisher will still take in the same $30 but they will need to pay Nintendo a licensing fee and a royalty fee to the developers. So their return on each additional copy of software sold due to advertising will be a lot less than Nintendo's cut.