As Mop it up pointed out, the install base is larger, now. Plus, NSMB Wii will have primed more people for the game.
If it doesn't outsell the first Galaxy, then it's not a system-seller at all, and using movie-based terms, I wouldn't call it a blockbuster, either. A sequel on the same system as its predecessor should outsell that predecessor, due to larger install base of the system. If it doesn't, it can't be as good as the original. This is, of course, assuming that the brand isn't destroyed by the first title, so that people don't buy the second one.
Since when has buying the first game in a series constituted buying the sequel? And even further how does not outselling the first galaxy even come close to questioning the quality of the game?
It's not that buying the first game constitutes buying the sequel. It's that, with a larger install base, it should be capable of selling at least as many copies, if equal in quality. Those who bought the first but don't buy the sequel should be offset by those who buy only the sequel, due to a larger install base.
Obviously, one must remember the distinction between the game's quality and its success. For instance, maybe Darkside Chronicles is a better game than Umbrella Chronicles... but Umbrella Chronicles came closer to being a system-seller (although it wasn't one). Just as a blockbuster movie isn't just a good movie, but a movie that breaks records at the box office, so too is a system-seller a game that goes beyond just being quality.
BlackNMild2k1 - Marvelous Interactive/Rising Star Games has been missed... although, their wall would be one of the few that I'd be tempted to call a "wall of <something other than shame>". Their games have consistently been niche, but their support of the Wii is clear-cut, and only one game on the list would fail to be considered at least decent (and even that one, I'm not sure of). Also, Atari hasn't been listed, nor has Disney, Hudson, Koei, LucasArts, or Warner Bros.