I dont know.
Here are a few simple obvious reasons.
1) Past MAD strategies fail when nuclear weapon or WMD technology is proliferated by third world nations to third parties who focus on assymetric warfare.
The ability of a terrorist organization to build WMD is ultimately linked to a piece of ground somewhere, where the devices can be fabricated. An obvious center of gravity which all Western nations should seek is to insure any party who fabricates are displays tendancies and appearances of fabricating those devices are held accountable in the international community.
Saddam Hussein repeatedly exhibited behavior and Iraq displayed behavior and actions rebellious of all international law and a continuous desire to control and/or deploy WMD while picayrunishly remaning deceptive about their actions.
Iraq was an obvious target to be toppled to retain international peace and respect for law and order within a community of nations.
2) Iraq for over 12 years continued to violate the UN conditions for the cessation of the Gulf War in 1991. They continued to disregard the No-Fly Zone and continued to shoot missiles at US and coalition aircraft patrolling the No-Fly Zone created by the UN. If for no other reason, the UN should have endorsed an entry into Iraq by force to seize control of all armed forces involved in those violations. This was basically a moot point though, probably one of the most blatent not iterated in the Iraq War debates initially in the war.
3) WMD are small and intelligence generally associates high security with their storage and movement since days of the Cold War. Unfortunately, the same country that can hide $650 million in cash, inside milk cartons in a farm house in Iraq, is the same nation that might have a small cadre of dedicated fanatics who,..if given merely hours, could relocate and hide WMD, even possibly out of the country.
4) Considering after the fact, we still don't have a body for either Osama bin Laden or S. Hussein, does this lack of evidence warrant a belief that those two individuals never existed and/or never even posed a threat to vital interests of the US? Such innocuous vital interests as life, liberty and pursuit of happiness of American citizens at the WTC were very well linked to terrorist associations in Iraq and Iraq's condoning of their training and behavior.
5) During the Iraq War, the level of terrorist activity within Israel dropped to an all time low sine 1964. Regardless if done for incorrect justification, we jerked some bad guy's chain somewhere and influenced and limited adverse responses to American interests and those of our allies. Those stats alone would have justified a Nobel peace prize in past Administrations.
6) Socialists around the globe need to understand that if they condone terrorist behavior, there will be irreversible risk for those positions and even if they control international bodies, such as the UN, they still won't control all risk.
7) If these reasons don't have import, then perhaps those who were willing to ignore the Baathist party atrocities, just need to have their butts kicked for G.P. (General Purpose). I.e. If they don't like it, TOUGH!