Because they did not trust the Republicans to leave as promised:

“Before, the Iraqis were thinking that if they sign the pact, there will be no respect for the schedule of troop withdrawal by Dec. 31, 2011,” said Hadi al-Ameri, a powerful member of the Islamic Supreme Council of Iraq, a major Shiite party. “If Republicans were still there, there would be no respect for this timetable. This is a positive step to have the same theory about the timetable as Mr. Obama.”

The legacy of the Bush administration is the belief that they, and by extension the United States, could not be trusted to uphold agreements.