Becase the Saudis are not in a position right now to arm the terrorists with weapons that can cause casualties on US soil that are several orders of magnitude greater than 9/11, but the Iraqis are in such a position. We are going to proceed to do what we have to do in Iraq soon, because otherwise we will inevitably have no choice but to obilterate Iraq in retaliation later.
Your reason for proceeding in Iraq is not very lucid. Saudi Arabia has PROVEN to be the source for terrorist activity against the US. Meanwhile Iraq has tenuous connections at best. By hitting Iraq we are not striking the enemy's head, merely one of the tentacles.