I think we probably thought Iraq would be easier than Iran. On the west, Iraq borders a friendly(ish) country and the countries to the north have their own problems. Iraq was also fractious (Sunnis, Shias, Kurds) in a way that Iran didn't seem to be (or at least less so, as shown by their performance in the Iran/Iraq war). Their people were more religiously motivated and more willing to fight than the Iraqis. We probably also hoped that we'd do Iraq first, democratic peace would break out, and we'd have that land mass (and all of its oil) to use against Iran later on.
And perhaps more importantly, it was a whole lot easier to trump up a war against Iraq than it was/is against Iran.
And perhaps more importantly, it was a whole lot easier to trump up a war against Iraq than it was/is against Iran.