They way I see it, third party games on Wii are between a rock and a hard place. The first problem is the Nintendo fan, it's not Nintendo or Zelda or Mario so it's shit. Just now someone asked me for a good Wii game, and I showed him the 8/10 review for COD:WaW. Maybe he should try it out. But he won't because it's not Nintendo or Capcom so it sucks.
The second problem is graphics. When I buy a COD game, I get it for the Xbox 360 or PC. I can get my Wii-remote shooter fix with Metroid and my FPS fix on PC so I don't need to experience 'mouse-like' FPS controls. So most multiplatform gamers will go for the Xbox 360 version and not the Wii verson.
The third problem is new IP's. Games like Zack & Wiki, didn't sell well because the games are not known to a wide variety of people. Resident Evil, especially Resident Evil 4 rings a bell but a title like Ghost Squad doesn't. I can only think of one company that tried to get attention, Red Steel and Rayman Raving Rabbids from Ubisoft. They were supposed to be 'the' third party launch titles when even the shoddy COD3 and NFS Carbon port outclassed them, not to mention SEGA's Monkey Ball game. People won't think of "Red Steel" the "Revolution Shooter" next, instead they'll think of the POS it was. Even if the sequel is amazing it's going to be hurt by this. That's why Ubisoft hasn't announced it yet, they know it's a dead series because the first game sucked hard.
On top of this issue there's the competition with Nintendo. You don't have to bother releasing your own platfrom game against Super Mario Galaxy. But Call of Duty could easily match, if not outclass HALO 3.