Why is it that people of african american, italian, hispanic, or indian backgrounds. Tend to have darker nipples, or darker in the private region?

Genetic variability. Those areas are darker not only in people of the above origin, but in caucasians as well, although, may not be as noticeable. Those areas tend to be "loaded" with melanocytes - cells that carry melanine, the dark pigment that determines a person's complexity. Some people have genetically more melanine than others.