Why does a fit persons heart beat slower than an unfit persons heart?

Because an active, physically fit person gnerally has a heart that does not have to work as hard (slower) as someone who is not in as good a shape and the heart has to pump, or work, harder to get the blood circulating. probably thelungs are in much better shape in the physically fit person and since the lungs and the heart interact, the lungs help the heart not work as hard.