In order for the USA to remain the defacto world superpower we need oil. Need it for our economy and our vehicles, need it for our military, need it for the host of products made from oil (tires for example.) Thus any significant source of oil must be maintained or the cost of the remaining oil will be effected.
If you don't believe Iraq is about oil you need to do some research into who we get oil from and how much it costs, and how much it'd cost minus a given source of it.
Iraq is only about oil. America isn't some white hat goodguy from the movies who does things solely because it's good and right to do. If there's no money or other incentive to doing things, we don't do those things. We never intervened in the genocide in Darfur. So trying to convince the American public we suddenly care about Iraqis is nonsense. We killed hundreds of thousands of them with economic sanctions alone (300,000 by conservative estimates.) Yet when one American gets killed we suddenly use it as a justification for war?
I'd have less of a problem with wars for oil if we actually won them once ina while. But since WWII we don't seem interested in decisively victories any more. Reason I think is war has become a business. And if you never go to war, that's bad for business. Megacorporations now provide everything top to bottom our military uses. No private peels potatos any more, some corporation sells them to the military now. And bullets, bombs, missiles all cost money. But if we never fire them at anybody there's no reason to buy new ones. Thus the motive behind these half-baked wars - deplete stocks, buy more. And if we can get some oil in the process, all the better.