Juan Cole explains why:
The old Sunni Arab power elite, mainly Baathists or the officer class, has not reconciled itself to the political ascendancy of the Shiites and Kurds. They still think they can destabilize the country and take back over. I would compare them to the Phalangists, the fascist Maronite Christians in Lebanon, who fought tooth and nail 1975-1989 against recognizing that Christians were no longer a dominant majority in Lebanon. Eventually they had to accept a 50/50 split of seats in parliament (which is generous to the Christians, given that Muslims are now a clear majority). That the Sunni Arab elite might be quicker studies than the Phalangists is possible but a little unlikely.