To understand why, you must first realize that Islam is not just a religion rather it is a way of life. This is why Islam has a built-in welfare system and pre-cautionary safe-guards. God knows that men have a higher desire for intimacy, they like to have more children and a wider company of wives. In societies where polygamy is not allowed, men take the re-course of pornography, prostitutes, extra-marital affairs etc.
In all these cases women are being treated as sexual commodities. There is no accountability and if in case of the mistress where emotional needs of women might be being met, that relationship is often based on deceiving other women, taking from her rights and a mistress can be thrown away and replaced instantaneously. Also, there is no limit on how many mistresses you can have no how many prostitutes you can visit. By women opting for polygamous marriages, they become someone’s wife, they are treated honorably, and they are loved equally.