There are third party Wii games that keep selling, so they are evergreen, just to a lower degree.
From what I've seen most of the third parties legs are from price cuts, the games sales dry up until they are reduced, whereas Nintendo can hold their price.
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, as well as the prejadise retailers have to non-Nintendo software on the Wii...