Because men are more assertive, more aggressive, and more likely to rise to positions of dominance. Far fewer women are interested in achieving leadership roles. And in many societies, leadership has been specifically a masculine role, with power passing from succesive male rulers to their male heirs.
However, even in the most patriarchal of civlizations, women have sometimes managed to achieve positions of power, usually through acting as regents for absent husbands, or for sons or nephews not old enough to rule. Ssome exceptional women have continued to rule even when the heir has reached the age of maturity, as with Hatshepsut in ancient Egypt, and the Empress Wu in Tang dynasty China for example.
