Foreign affairs analyst Gwynne Dyer says it's to cover up his entanglements with the United States.

An excerpt from Now magazine:

Dyer notes that Saddam was hung after being convicted in the deaths of 148 people in the Shiite town of Dujail in 1982 (there had been an assassination attempt on him there).

Dujail? Here is a man who began his career in power in the late 60s by exterminating the entire (mostly Shia) leadership of the Communist party in Iraq, went on to launch an invasion of Iran in 1980 that cost up to half a million lives, massacred his own Kurdish population in 1987-88 when some of its leaders sided with the Iranians, invaded Kuwait in 1990 and massacred Iraqi Shias in 1991 when they rebelled against his rule at the end of that war. And they hanged him for Dujail?

It's as if they'd taken Adolf Hitler alive in 1945 but ignored his responsibility for starting the second world war and his murder of 6 million Jews and just put him on trial for executing people suspected of involvement in the July 1944 bomb plot. With all of Saddam's other crimes to choose from, why on earth would you hang him for executing the people suspected of involvement in the Dujail plot?

Because the United States was not involved in that one. It was involved in the massacre of the Iraqi Communists (the U.S. Central Intelligence Agency gave Saddam its membership lists). It was implicated up to its ears in Saddam's war against Iran to the point of arranging for Iraq to be supplied with the chemicals to make poison gas, providing Baghdad with satellite and AWACS intelligence data on Iranian targets and sending U.S. Air Force photo interpreters to Baghdad to draw Saddam the detailed maps of Iranian trenches that let him drench them in poison gas.

The Reagan administration stopped Congress from condemning Saddam's use of poison gas, and the U.S. State Department tried to protect Saddam when he gassed his own Kurdish citizens in Halabja in 1988, spreading stories (which it knew to be false) that Iranian planes had dropped the gas. It was the U.S. that finally saved Saddam's regime by providing naval escorts for tankers carrying oil from Arab Gulf states while Iraqi planes were left free to attack tankers coming from Iranian ports. Even when one of Saddam's planes mistakenly attacked an American destroyer in 1987, killing 37 crew members, Washington forgave him.