Several causes. Women generally have lower peak bone mass than men. At menopause (which men usually don't experience) the fall in estrogen levels leads to accelerated bone loss. They start at a lower level and lose faster -> more osteoporosis.
Estrogen level chang. Hormone changes estrogen decreases and women loss bone mass.
Menopausal bone loss. The biggest change in bone density between men and women occurs in the years immediately after menopause. The combination of a lower peak bone mass, accelerated bone loss after menopause, and longer life span means more women are diagnosed with osteoporosis than men.