Corruption didn't do all that well either. This has nothing to do with the quality of either title, and it certainly isn't a result of the crappy writing, voice acting, or any other specific criticism you want to make.
The reason is actually pretty simple, and I'm surprised that Nintendo of America claims to be so puzzled. Metroid can't sell a million units for the same reason that so many other high-quality, complex, adult-oriented games have performed below expectations on Wii. Nintendo didn't establish the platform as a strong destination for such games in the first place. Before Other M, what was the last first-party Wii game with a similar level of design complexity and seriousness of tone? All I can think of is Corruption itself, released three years earlier. It's no wonder that gamers aren't looking for these kinds of titles on Wii!
I agree. The system is designed (and marketed) towards group play. 'Sharing' the gaming experience. Looking at the sales charts, you'll probably struggle to find many single player games that rank high on the list and don't have Mario in the title. Actually, it kind of surprises me that we don't have more of those types of games developed for the system. Say for example, good 4 player racers. Racing games are a blast in a group, and I believe Mario Kart was our first real offering.
That being the case, I think games for Wii are driven by what folks experience while playing in those groups. Tried, Wii Fit at a friends house, gain knowledge of it, buy it. Heck, that's how the system took off. Good games that cater to that group play will strive in the same.
Another typical way for games to catch on if for owners to show them off. In example, I was introduced to Shadows of the Colossus that way, because it was a neat concept. Word spreads, kids take it back to parents, and the next thing you know parents are asking for it in store, even if they don't have the right system to play it...
Metroid M has neither of those points AND it's fairly straight forward, of moderate length, and linear. If someone I knew was over and was interested in the title.... I'd probably just let them borrow it.