It's not easy to sell games on Wii.
The market is crowded, and mostly with junk developed by 3rd parties. Trying to make your game pop and be something people want to buy takes real effort and effective advertising. It also requires that development studios be creative, making sure people can clearly understand the concept and goals of the game.
Wii owners are not a predictable and easily identifiable demographic. You can't just take an existing game or genre, combine violence with some T&A action for young males, polish the graphics until they shine, and instantly release a top seller on Wii. (However, Bayonetta seems to be doing fine on PS3/360.)
There is serious competition from first-party titles. Nintendo releases quite a few games, and most of them are proven keepers. People know Nintendo games are good, make a point of buying them, and keep/play them longer than the average PS3/360 gamer does first-party titles on those consoles. Nintendo is playing for keeps and big money, so if you aren't ready to bring the A-game then it might be better to stay home.
Controls and programming are unique, and may require more tweaking depending on what type of game you are creating. I have heard rumors that Nintendo's system documentation isn't as good as it should be, which might explain why some developers have issues here (or when using other system-specific features).
So with all those factors in mind, do I have sympathy for developers who aren't turning a quick profit on Wii? Enh... a little bit. Sometimes. Mostly they've created their own problems though, and retail failures can often be directly attributed to developer (or publisher) decisions.