Ackerman gives some background

Spencer Ackerman, who knows about as much about Iraq as any journalist, offers some important background on what has brought Maliki to this point. In a nutshell, "Unconditional Surrender Bush" again greatly overreached.

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...When those negotiations [to extend the U.S. occupation beyond the UN mandate] began, the U.S. reportedly presented the Iraqis with terms so breathtaking that they'd embarrass Lord Curzon. Bush wanted unilateral control of Iraqi airspace; legal immunity for all U.S. troops and contractors; the unilateral right to arrest and detain any Iraqis his commanders desired, and for unspecified periods; and several military bases. When Maliki indicated discomfort over acting like Gaius Baltar on Occupied New Caprica, Bush gave another indication of his "friendship and cooperation" -- blackmail.

All this came in a political context that Bush was either unattentive to or dismissive of. Despite spotty media coverage in the U.S., the deal prompted a massive backlash in Iraq, where basically every organized political force not part of Maliki's government rejected it. Maliki's allies were likely to lose the looming provincial elections already; now he had given them the albatross of clear collaborationism. And something similar was at work in the U.S.: the candidate with a clear and consistent history of opposition to the Iraq war won the Democratic primary, while the Republican candidate backed an endless occupation that he said might last a hundred or even a thousand years.

Maliki is a politician after all, and saw no upside to surrendering his country to a permanent occupation upon the demand of the most unpopular POTUS in history. And as Spencer points out, Maliki's down side might be severe or even fatal, "Maliki surely thought, if I sign this deal, my people will run my body through the streets and hoist me from a fucking lamppost. Not that the electricity works, but still."

(HT to Andrew Sullivan)