I think they want to preserve as much road as possible. We'll need that road when we get there.
Approximately half of the total energy generated will be used in swelling the bomb casing to 1.5 times its normal size prior to fragmenting and then imparting velocity to those fragments. The remainder of this energy is expended in compression of the air surrounding the bomb and is responsible for the blast effect. This effect is most desirable for attacking walls, collapsing roofs, and destroying or damaging machinery. The effect of blast on personnel is confined to a relatively short distance (110 feet for a 2000 pound bomb). For surface targets blast is maximized by using a general purpose (GP) bomb with an instantaneous fuzing system that will produce a surface burst with little or no confinement of the overpressures generated by excessive burial. For buildings or bunkers the use of a delayed fuzing system allows the blast to occur within the structure maximizing the damage caused by the explosion.