WMD WTF?

Speaking of acronyms emerging from the distant past of the Bush 43 administration, here's a fascinating story in the New York Times today:

The Central Intelligence Agency, working with American troops during the occupation of Iraq, repeatedly purchased nerve-agent rockets from a secretive Iraqi seller, part of a previously undisclosed effort to ensure that old chemical weapons remaining in Iraq did not fall into the hands of terrorists or militant groups, according to current and former American officials.
The extraordinary arms purchase plan, known as Operation Avarice, began in 2005 and continued into 2006, and the American military deemed it a nonproliferation success. It led to the United States’ acquiring and destroying at least 400 Borak rockets, one of the internationally condemned chemical weapons that Saddam Hussein’s Baathist government manufactured in the 1980s but that were not accounted for by United Nations inspections mandated after the 1991 Persian Gulf war.

This is parsed as a rather sneaky action on the part of the CIA, but to me the larger question is this: if Saddam had 400 warheads loaded with sarin gas, why hasn't that fact been announced as justification (if only partial) for the 2003 invasion that deposed him?

Supposedly they were old, but an unspecified number of them contained potent sarin gas, and were apparently modified to be used against American or post-Saddam Iraqi forces or civilians:

These munitions were remnants of an Iraqi special weapons program that was abandoned long before the 2003 invasion, and they turned up sporadically during the American occupation in buried caches, as part of improvised bombs or on black markets. 

The warheads were intended to be fitted to Russian "Grad" truck-mounted, multiple-launch missiles, for use in the Iran-Iraq War of the 1980s, the one of which Henry Kissinger said "It's a pity they can't both lose."