Nintendo did this the same thing last gen with their first party Gamecube games, and part of the reason I didn't even bother getting a Wii. Their first party games, which are probably the only ones I'm interested in, almost never come down or even as a Player's Choice it's more expensive than the other ones.
This game won't come down in price cuz it's pretty much the game to get for the system, definitely one of their bestsellers. It's totally worth the full price though, there's so much to do and it's extremely fun. Nintendo doesn't loose potential customers cuz people know this game is a must-have so they get it. If you want it for less, buy used. If it didn't sell so well or they released a new one on the same system then the price would come down. Don't be upset cuz its its a few dollars above $20, its not like it's a necessity.