What causes certain parts of the body to get darker/produce more melanin?

Any part of the body that gets direct exposure to the sun is more likely to experience skin discoloration, whether the face, forearms, hands, neck, or legs. But discoloration can appear everywhere on the skin, since it’s caused by so many potential factors—including what some derms believe is caused simply by friction from mobile areas of the skin around joints. If darker skin patches appear with a velvety texture in folds and creases of skin, usually in the neck, arm, and groin area, it may be something called acanthosis nigricans. For more information, see here.

References:
Frictional Asymptomatic Darkening of the Extensor Surfaces by Krishnamurthy S,  Sigdel S, and Brodell RT. Cutis.