Men strive to be the alpha and are in constant competition with each other. Men being as competitive as they are makes them uneasy in being in a polyandry relationship. Then again not all women are open to a polygamous relationship because of the fear of losing attention and affection of the person who they married to another person.
Personally don't think I could do a polyandrous relationship because I assume there would be lots of unnecessary competition to be "number 1".