Women out live men and thus in the older age group there are more women than men alive and thus more women than men in all places including cruises.
Women being on average the physically weaker sex are more concerned with security and safety then men. A cruise is safer than say renting a car and going to unknown places. With a cruise everything is right on the ship and one actually doesnt need to leave the ship at all. Even in port it is possible to take an excursion or taxi right from the ship (though excursions from the cruise line tend to me more expensive then the same activity not through the cruise line).
Though times are changing it is still more common for the husband to work but the wife not to work then the other way around, thus a wife is free to go on a mid week cruise with the children while the husband works.