Why do they refer to Saddam Hussein simply by his first name?

"Saddam Hussein" isn't even his full name; it's "Saddam Hussein al-Majid al-Tikriti." The first name is his given name, the second is his father's name, the third is his family name, and the fourth is the name of his extended family who comes from the Tikrit region.

In 1968 when the Baath party took over with Saddam as vice president, it outlawed the use of tribal names. The government told its people that they (the citizens) owed their allegiance to the state rather than to a local tribe. So Saddam was left with "Saddam Hussein." Later, he chose to be referred to simply as "Saddam," since he wanted to be considered the Iraqi people's Grand Uncle. Now, virtually every Middle Easterner mentions him by just his first name. That is also the primary reason why news organizations print and say "Saddam."

When President George H.W. Bush invaded Iraq in 1991, newsrooms argued over which name they should use for Iraq's president. The problem was that King Hussein of Jordan had the same last name, and no one wanted to confuse that country's leader with the brutal dictator Saddam was. So some just referred to him as "Saddam," including the AP, Reuters, and the BBC, since that's what Arabs called him anyway. Those same organizations still mention him by his first name. Others, like The Washington Post and the Los Angeles Times use "Hussein." The New York Times uses "Mr. Hussein," following its obsessive tradition of honorifics. The Wall Street Journal printed "Saddam Hussein" in every reference to him until the king's death. Now it also uses the honorific.