because of the physical properties of the atmosphere of the moon and the material it is made of... the loose particles on the moon would have blown away by the rocket, and the hard rock beneath is to hard to get 'blasted'...
The downward traveling exhaust stream would impact the ground and rebound mostly outward and away from the surface. Since there is no atmosphere to interact with, the gas molecules would simply fly off and disperse (see note below). The only dust particles that would be displaced would be those directly impacted by the exhaust gas. Since the exhaust stream was concentrated mostly in the area directly beneath the Lunar Module, this zone would experience the greatest disturbance. The area adjacent to the LM would be largely unaffected by the exhaust stream.
NOTE: On Earth, the exhaust gas would impact and displace air molecules that would, in turn, displace other air molecules and so on. This phenomenon would create a large area of disturbance. Since the Moon has no atmosphere this type of widespread disturbance would be nonexistent.
but the footprint...
"The lunar surface is predominately composed of materials that fall under the general category of silicates. Silica has a natural tendency to bond with other silica, forming large molecular chains. When a meteoroid impacts the Moon, much of the energy goes into fracturing the surrounding structure causing breaks in the molecular bonds. On Earth, these "exposed" bonds quickly fill with oxygen in a process called oxidation or weathering. On the Moon, with a total lack of oxygen, these bonds have nothing to attach to until an event occurs that aligns the molecules. When an object, such as an astronaut's boot, disturbs lunar dust new molecular bonds are created. The new bonds enable the dust to hold its shape, forming an impression of the deforming object. Thus, footprints can form despite the absence of water. "
or if ya don't like reading...