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.
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.
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?
Generally, if more eumelanin is present, the color of the hair is darker; if less eumelanin is present, the hair is lighter.