How did monogamy come to be the most prevalent legal model for relationships?

Monogamy wasn’t always the predominant relationship mode in the majority of societies in the world, according to Wikipedia and other research. At some point in time, it was estimated that 80 to 85% of societies sanctioned polygamous marriage. However, even though it was the predominant model, anthropologists estimate that only 20% of men were able to afford more than one wife. At some point, perhaps in the last five hundred years, it became the dominant model. When? How? Why?