Why do I have a naturally slow pulse?

Fitness. People who exercise on a regular basis will tend to have a lower resting heartrate than others -- as long as it is not symptomatic (causing you to get light headed) it is probably normal.
Heredity. Some are just born that way. Some take medications which reduce the pulse and others exercise a lot which also tends to reduce the resting heart rate. Lastly some people have heart disease which can lower the pulse rate.