If the main reason was regime change, then:

Would you want a foreign power to change the US leadership and democracy, forcefully?
How would you feel about that?
Put yourself in the place of an Iraqi who's going through this , right now.
Saddam is not even completely gone; he could come back, but there is a foreign occupying power that doesn't even speak his language, now. And this means that occupying power doesn't trust you. it humiliates you.
This is why international law cannot support forceful occupation of a country unless there's a threat to neighbouring countries which wasn't the case. This is why WMD's needed to be justified for the UN.
Now, regime change means another regime, doesn't it? Which one? a pro-american one or one the Iraqis want? I'm absolutely sure that they cannot match completely. Partly, yes, but not completely.

Raoul