easy answer if u ask me. they didn't just want to invade iraq. they wanted to put an american stronghold on the middle east. after thousands died on 911 america cheered the shock and awe. not because we found a WMD but because we lost lives in an attack.
Non-answer if you ask me. As long as they (whoever "they" are, and there would have had to have been hundreds of "them") were going to mastermind a big dupe, why not make the patsys Iraqis? "They" then wouldn't have had to waste their time and resources in Afghanistan, and would have gotten near unanimous support for an invasion of Iraq. Which, despite your assertions about America cheering, was hardly what happened.