There are two basic reasons why concrete cracks, either by an external applied force or a change in volume.  It is interesting that a change in volume will not cause cracking unless the concrete is restrained.  If there is no restraint, the concrete will just shrink (pull together) and not crack.  The restraint usually comes from the sub-grade but may also come from surrounding walls or previously poured slabs.  The footings incorporated into monolithic slabs hook the sub-grade, increasing the restraint on the concrete.  Thus, monolithic slabs tend to crack more than traditional slabs with stem walls.