There's a simpler reason, essentially the "follow the money" argument: It would have been A LOT cheaper (literally $trillions) and A LOT easier to quietly carry on supporting Saddam's regime in exchange for cheap oil. Hell, he could probably even have been convinced to hand over some inconvenient Shia "terrorists" and be applauded as a responsible ally in the war against terror.
On the other hand, the then-US government and its advisors clearly didn't read as much as General Mattis (or read the wrong books), and did not expect to find themselves embroiled in a bloody civil war. The fact that, as usual, there is not one bad single reason to go to war, but multiple ones, does not mean that acquiring strategic oil supplies was not a significant factor. And of course, spreading freedom and Coca-Cola through shock and awe.
