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...
Yeah i mentioned that, price cuts make my argument irrelivent. At full price Nintendo games continue to sell, and some of you are right, they have always continued to keep selling for ages. Casual is not an excuse.
“When we make some new announcement and if there is no positive initial reaction from the market, I try to think of it as a good sign because that can be interpreted as people reacting to something groundbreaking. ...if the employees were always minding themselves to do whatever the market is requiring at any moment, and if they were always focusing on something we can sell right now for the short term, it would be very limiting. We are trying to think outside the box.” - Satoru Iwata - This is why corporate multinationals will never truly understand, or risk doing, what Nintendo does.