I dunno. Same reason we don't have enough people to stand up and put a stop to the foolishness that's going on in Washington, I suppose. Fractionalization, Factionalization, and lack of a common goal. A few have always been able to terrorize & tyrannize the many.
The reason we don't do anything about the foolishness in DC is simply because it's not important enough to us. That's another way of saying what you said. So, as long as ridding the country of the insurgents is quite important, the country can be united behind crushing the insurgency. Right now, they don't view it as important because we're taking care of it and because they believe the insurgents only hate us and not the Iraqis. If we leave, we can show them that they are wrong and they will be forced to crush them-- or perhaps they are right and the insurgents will leave. Either way, though, we win.