This is because America wants to destroy only a small part of Iraq, Hussein and his cronies who turned against America 15 years ago, they want the rest of Iraq to join in the American project to build a global free market utopia.
This confirms to me the motivations for the war, that i suspected before it started. America's single aim in any issue is to strengthen and expand the global capitalist free market economy of which American firms and their CEOs/owners are the main beneficiaries. Iraq is particularly important because it was formally counted by the US as a 'good guy' and on their side in this aim (IE opposed to Iran and The Soviets) and also because they have an abundance of one very important commodity - Oil. What America wants is for Iraq to rejoin their campaign to build a global free market utopia, and also to take revenge on the man they hate for deciding to leave it. This is what America wants from all nations and people, usually they get it without much of a fight, but some countries, such as Iraq,Afghanistan or North Korea just don't want to join in. For them military force is used (justified by some more PR friendly cause - such as Terrorism or WMD), to destroy those who are unpersuadable and persuade those who are suffering under harsh regimes to join 'willingly'. There is no doubt that many in Iraq will join this global economic utopia willingly, even enthusiastically, because it is far better than what they are used to, and like the many millions of other people around the world who gobble up American culture and enthusiastically play in the casino of capitalism, they hold out the hope that they can stay among the winners.