Not that I'm taking sides, but there is an answer for your question.
Army grunts working REMF jobs don't get in the way of combat troops like civilian contractors can, are more expendable (lets face unpleasant facts) than civilian contractors, and are cheaper. Your Army grunt in REMF draws roughly the same salary in Iraq and Afghanistan as he does in the U.S., but convincing a civilian contractor to leave the U.S. and go to work near the front lines in battle areas costs bucks.
That being said, it might still be a good idea to replace grunts with contractors.
But understand that it's going to cost money, that you can't move the contractors on forced marches (sometimes necessary in evolving combat), and that the civilian contractors are going to engage in civilian activities that might not be entirely conducive to conducting a fighting war.