My parents are divorced
Probably because monogamy is required by law.
Where is it required?
Monogamy ( ) is a form of relationship which required by law most places around the world. Plus who has time to be with more then one person. Hippies. 
Is polygamy illegal? 
Yes, polygamy or multiple mating is illegal and gross. 
Well it being gross is an opinion.
Polygynandry
Polygynandry is a mating system in which both males and females have multiple mating partners during a breeding season. In sexually reproducing diploid animals, different mating strategies are employed by males and females because the cost of gamete production is lower for males than it is for females. The different mating tactics employed by males and females are thought to be the outcome of stochastic reproductive conflicts both ecologically and socially. Reproductive conflicts in animal societies may arise because individuals are not genetically identical and they have different optimal strategies for maximizing their fitness
often on the death of the first spouse or after divorce; they cannot have more than one spouse at one time because that would be polygamy which in countries with marital monogamy like the US is called bigamy. Polygamy is a form of marriage in which someone marries multiple people at a given time, and is illegal throughout the U.S. under the Edmunds Act. Part of the function of looking at marriage from a sociological perspective is to give insight into the reasons behind various marital arrangements.
Sociology of marriage Reasons for marriage.