The reason games on the Wii sell better over time (and consistently) is a combination of its higher userbase (70 million) and the fact that gamers on the Wii don't buy into 'hype'.
Whether its a game like Mario Galaxy or Blom Blox, the only people going out there 'day one' to buy the game are hardcore gamers or people like us, who know when games come out first day (usually, they're one and the same). However, its obvious that the vast majority of Wii owners are either not hardcore gamers or own Wii as a secondary console. So they aren't rushing out there day 1 to buy Monster Hunter Tri, Punch Out!! or Muramasa: The Demon Blade. But...they do eventually.
This 'I'll get the Wii game eventually' mentality causes a lot of Wii games to look like they have really good legs. While in effect, they're just making up for the fact that they didn't sell 2-6 million on day one like their major HD counterparts who put tons of hype and advertising into each game.
Six upcoming games you should look into:
