Continued marketing definitely plays a role, although with the likes of Mario Kart DS, which I doubt has seen an advertisement in ages, is still somehow making NPD top 20 lists, there are other factors as well. Obviously game quality has something to do with it too, but also very important is the type of game and the audience it's attracting.
Thinking about the sort of games that the "traditional" gamer typically enjoys (the things that tend to sell very well on other platforms), they follow a similar sort of archtype that those players are familiar with. But the thing is, these gamers are also fairly specific in their tastes, and it doesn't help that there is so much competition within those genres by other game developers and publishers. If, for instance, you were a gamer that had paid attention to or noticed the advertisements for Darksiders, a mature action title with mythological influences, you might go out and buy it relatively quickly. On the other hand, if you didn't keep up with that initial hype in the first month, what incentive would you have to buy it even a month or two later, by which point you're seeing marketing for God of War III, a mature action title with mythological influences?
Now instead take a look at something like Mario Kart Wii. A month later, were we seeing other top-notch online arcade-based kart racing games? If someone in, say, November had just picked up a Wii and wanted a racing game, what other alternatives are there? This doesn't even take into account the inherent popularity of the Mario name, or the fact that people may have experienced the game previously, at a friend's house. The same goes for a lot of the Wii stuff in general--a lot of these "evergreen" titles, not counting Wii Fit, are designed to be played and enjoyed in the company of other people, which is not something you can easily do with the likes of the FPS, action games, or RPGs that tend to pop up more often on other systems.
So I guess, if you combine the social aspect of Wii along with a comparative lack of competition for a lot of these games, that's a good place to start looking at least.