The need for armor is largely psychological. 1/4 inch plate welded to the side of a truck may provide a measure of protection against 7.62 x 39 but won't do anything if an IED made from a 155mm shell goes off near the truck. In the case of RPG attacks, the armour skirt may do more harm than good by increasing the target area of the truck.
Furthermore, adding armour reduces the payload rating of the vehicle making additional vehicles necessary to haul the same amount of material. The additional vehicles or additional trips just increase the odds of being hit.
In the movie "To Hell and Back" there is a scene where a tank commander is complaining to the ground pounders troops about how thin the armor is on his Sherman compared to a Panzer or a Tiger tank...
One of Audie Murphy's buddies replies..."Yeah?....well....just how thick do you think this GI shirt is?"