I think a lot of third parties are seeing the success of Wii Sports and Wii Play and think the most lucrative and safe games to be developing for the Wii are casual ones and those aimed at a younger audience while those developing for the 360/PS3 see the success of games such as Gears of War and Resistance: Fall of Man and so make games targeted at a similar audience.
And since third parties have finite resources they choose to put them towards the most lucrative audiences for the games (eg. Ubisoft making Haze for the 360/PS3 and Rayman: Raving Rabbids 2 for the Wii). Would it make sense to port some of those casual games or those for a younger audience to the 360/PS3 when similar games in the past have done horrible, or to spend the money and resources to adapt Haze so it worked well with the Wii without the guarantee of similar sales to those on the 360/PS3?
Will games like Metroid Prime 3 change that? Possibly to some extent but games like Halo 3, GTAIV, and Call of Duty 4: Modern Warfare will still likely convince third parties to continue making games targeted at that audience on the 360/PS3.
