Strategypage notes this as well as explaining why we are successful:
One major accomplishment in Iraq and Afghanistan has been the tight security of U.S. bases. There have been only a handful of security breaches (suicide bomber getting inside). This saves lives, and also provides the troops inside these bases with a respite from all the violence outside. This is great for morale, and helps reduce combat fatigue.
Strategypage goes into the reasons for our success. The enemy failure to inflict casualties on our softer rear echelon troops in their bases is an important factor in our lower casualties this war. When the enemy has to fight our frontline troops, the enemy dies.