Irak, with it's stable government, large population, large army inherited from the war with Iran, and natural resources, was a strong political player in the middle east, and not quite aligned with the USA (for instance, being able and willing to threaten its closer ally Israel).
So maybe the reason was just the desire to clip those wings?
It's such a coincidence that Israel is a prosperous (and very aggressive) country and all its neighbours either keep a very low profile or are reduced to rubble... By the US, and all for the sake of democracy of course.