StrategyPage notes we're blessed with stupid enemies.
The war has become 95% of free Iraqis vs. about 5% who want to put themselves back in charge. The "insurgents" are utterly bankrupt of ideology; they don't even pretend to be working "for the people" like leftist insurgencies during the Cold War did. Their philosophy is "we'll kill people till we get what we want."
And why shouldn't they think that way? Hey, it was how Saddam ruled the country for decades. I guess there's a kind of inertial stupidity working there. They don't seem to realize that Iraqis aren't pining for the good old days, and are lining up for the chance to die for democracy and freedom. When your best strategy is to attack the long lines at recruiting centers for soldiers to fight against you, that should tell you something about the popularity of your movement and the likelihood of its success. Especially when the lines re-form right after you bomb them.