Answer:
When a car gets stuck in the mud, two things are at work. First of all the mud is very slick. The tires of the car have no traction and simply spin helplessly trying to move the heavy weight of the car. Secondly, mud, if it is deep, has suction. It literally sucks the car's tires, holding the car in the mud. Combine this with mud's slickness and your car is literally stuck.