Polygamy is becoming more difficult. Supporting a large family in a cash based economy is very expensive. It worked far better in crop based economies where getting an extra wife meant one more person to work in the fields, plus more children.
Polygamous marriage is very unstable in places where women have basic civil rights like education, property ownership, divorce, and the right to work outside the home. Women with options simply do not like living as some old goat's 4th wife.
Polygamy is a strain on a country's economy. The women generally are not allowed the right to work for pay, and polygamy creates a lot of unmarried men with no community ties. In a free country, those people are generally contributing more to the society.
May I remind you the polygamy is illegal in the United States.