The Wii's userbase is much more "Mainstream" than the other two consoles, and this means that the typical user is probably less likely to go searching on the internet to find obscure high quality games than other consoles; and even the more "Hardcore" userbases of the PS3 and XBox 360 do not have that many people who would be willing to do this.
What this means is that the brand associated with a game, word of mouth (and first hand experiences), and marketing are essential to the success of a Wii game ... Third party developers who have released games that have (at least) two of those qualities have seen decent sales or better. Unfortunately, far too many developers decide to drop brand new IPs on the Wii with minimal or no multiplayer gameplay on the Wii, and don't support it with any marketing which results in a game few users know about; and when they see it on the shelf they would not be likely to buy it.
