Desperate measures to save Baghdad suggest a desire not to allow the desperate to develop.
Better to stop the enemy early rather than TOO late.
Realize their command and control would have been less than stellar, if even operational. They'd likely need to use WMDs at chokepoints just so they could have the correct assets in place where coalition troops would have to go to complete the invasion. With no air cover, moving the weapons after the war had started would have been extremely high risk.