response: I don't know why large numbers of them do, but I know that in general eumelanin controls how dark hair is. The more there is, the darker the hair.
original_response: Well it depends on the level of eumelanin.  If there is more, the hair color will be darker and vice versa.  Brown just happens to be in the middle.
history: I just brown hair.  I wonder how many people have brown hair. Next to black hair, it's actually the second-most common hair color. Is there a reasons so many people have brown hair?
knowledge: Generally, if more eumelanin is present, the color of the hair is darker; if less eumelanin is present, the hair is lighter.