Because women don't want to share thier husbands. Is that simple enough?
Make all sorts of assumptions about the nature of women or a woman's character, but most women don't want any other woman near her man, whether she's flirting/sleeping with, or marrying him.
You are confusing affair with marriage. Marriage is a legal and legitimate bond between man and a woman whereas affair is something else. Moreover, in today's society, what I have noticed about Polygamy is that it is not only the women who oppose it but men also have same type of view point So it defeats your point of view saying that women's character is weak that she won't go ahead with Polygamy or will not allow it as the society in general be it man, his family and people who know him will disapprove of it too.
I don't understand why some people choose to ignore the mongomous, most beautiful relationship our Prophet S had with Hazrat Khadijah R.
The reason that polygomy is gone in today's society is because most women today are not willing to marry into that kind of relationship and don't feel the need to, they are able to support themselves, and hve enough protection from their fathers and brothers.And because they're are many good men (allhumdullilah) who are able to respect that wish of their wife and are not worried about the quanitity of the children they produce, but the quality of the children.
Mongomous relationship is beautiful and it is preferred, but let's not look down upon people who practice Polygamy or have thought of it as there is nothing wrong with it.
You make a very good point that as women today are educated as well as can make and bring money to home. They are not entirely and solely dependent on their Husbands. They can make it on their own, which gives them more choices and does make it less likely for Polygamy to exist in a society.