With Call of Duty I guess Activision figures it's worth it. Call of Duty is like the biggest franchise ever in terms of $, so it can probably support the costs of adapting the game where other games might not be able to.
As for Dead Space: Extraction, it's an on-rails shooter, not a semi-non-linear game like the others. Also, it's entirely possible that they decided to do an on-rails shooter and chose the Wii for it's light-gun-like controller rather than saying "We need a Dead Space game for the Wii, so come up with something".
Any time a company is deciding whether to release a game on multiple platforms it has to do a cost/benefit analysis, and with the Wii the fact that its capabilities are so much different really drives the cost side of that up. That's not to say that it won't sometimes be worth it, just that it's less likely than with more comparable systems.