A large convoy is just asking to be attacked. Remember, a lot of these militant fighters show little regard for their own life - if you've got a huge convoy of troops going around, and then some guy with a pretty powerful bomb strapped to his back comes along and blows himself up, you're going to kill/maim a lot more of the soldiers.
What happens when the public find out about this, and sees pictures of firstly a load of Iraqis getting slaughtered by these militants, and then the US army mowing them down? There'll be an outcry. Remember, this is a politically biased war, more than anything else, and public opinion plays a majorly large role in what's going on out there.