Because this is an ingrained biological imperative for men, and not for women. Women can get pregnant and have only one persons child at a time while a man can impregnate multiple women. And this is from a purely biological standpoint from which neuroscience and anthropology is based. Neuroscientists have proven that Monogamy is biologically unnatural for Humans because the parts of brain in some mammals which are responsible biologically for Monogamy are absent in Humans. From memory i remember there are only 5% of mammalian species are strictly monogamous and Humans are not one of them. The primordial structure of a pairing between an alpha male (genetically and spiritually superior) without any social conventions is a primary partner as well as multiple side partners. There was a lot of intraspecific violence between men and Pagan societies that were understanding of the natural truth ,tried to placate the genetically superior males with the caste structure. For example, in the original vedic system ,the society and allowance of wives was allowed as such,
Brahmins (meditators,spiritual teachers) - upto 4 wives
Kshyatriya (warrior class,management,upper royalty) - Upto 3 wives
Vaishya (traders,buisnessmen) - Upto 2 wives
Shudra (laborers,menial workers) - Upto 1 wive
This was only given as a recommendation ,and not enforced. Economics and status played a big role in this. People could do as they chose. In many parts of Tibet where the women are extremely low, there is a system of multiple men marrying one wive. This happened in the Indian epic, Mahabharata when 5 pandava brothers had one common wive, Draupadi ,although some brothers also had another side wive.
However people generally did as they wanted. Rama the great ruler of the Sun Dynasty in the vedic period chose to only have one wive while his father had 3 wives.
Monogamy is a socially enforced behaviour that leads to stronger and more productive communities and stronger familial units that can be better managed ,when seen from the perspective of the state. However from a biological state of affairs it is not natural. Women will get attracted to you when they know that you've been with other women at almost an instinctual biological level (women like married men). If you ask they ,they will deny it but studies have shown that the Anthropological explanation is correct and women operate on the level on preselection i.e they like men that other women also like. The complete opposite is true for men.
This is all biological which cannot be changed and society is based on managing all this. The pagans had a more naturalistic way of life while the christians and muslims have repressed the biological tendencies of humans by enforcing monogamy. Todays hypersexual culture is the result of yehuborim manipulating the emerging true sexuality of humans after the fall of abrahamic religions into a totally destructive way. This is the root cause of feminism and all this other cult structures like swingers and open relationships etc. Its shown that the most staunch feminists actually want to be dominated by masculine males and have more bdsm fantasies than any other group of normal women. This is because of the big cognitive dissonance in their minds of ideology vs biology. In a sexually dimorphic species the bigger counterpart sexually dominates the other. That is true regardless of what species your talking about. A lot of problems will be fixed if you just naturalize the polygynous structure again in society. But only the best of the best will be able to manage multiple women as a leader. Its not for the faint of the heart. Its not for emotional and feminine men. Only a true warrior monk will be able to handle that much sharpnel,jealousy,friction without flinching.