1. Men live shorter lives (this has been the case since modern medicine eliminated what had been the leading cause of death in women: childbirth complications)
2. In many cultures women are not valued, and female babies are frequently aborted (not a problem among frum Jews, butg a serious problem in several countries including China and India)
3. Wars sometimes increase the unbalance, but many countries have taken to killing women and children, which of course reduces the problem (e.g. after World War II, Germany had serious shortages of males since the Allies killed many German men but few German women, but among Jews this wasn’t a factor since the Germans murdered women equally with men)