Infinity Ward is actually the one's that finished optimizing the Call of Duty 4 engine for the Wii. After IW finished optimizing the engine for the Wii, they handed it over to Treyarch so they can get familiar with it and get their hands in it. In this process, Treyarch worked around the things they wanted in their game and what couldn't make it in the game with the engine. And some of the things were cut due to time and cost and to Wii hardware.
Do you know what confuses me. Why do people say "Infinity Ward hates the Wii?" They clearly don't and I have talked to IW's Robert Bowling (FourZeroTwo) on many occasions and asked him on his thoughts about the Wii. He says "It's a great console, with a lot of potential." He has no problems with the Wii whatsoever. Infinity Ward loves the Wii.
Why would Infinity Ward finish optimizing their engine for the Wii, then never release a game for that particular console?
And the reason why Call of Duty 4 never came to the Wii was because Infinity Ward was trying to port the engine over to the Wii but sadly they didn't have enough time to do so. Activision said they HAD to release the game soon so they never got to porting the engine over. And in their down time after the release of Call of Duty 4, Infinity Ward finally got the chance to finish their already work-in-progress engine port, and then later gave it to Treyarch for World at War. And since the engine is now finished and is ready to be optimized even farther by Infinity Ward, there is no excuse of why Modern Warfare 2 shouldn't release on the Wii.