Thanks for your question Nurayn. Here is a detailed answer to it as contained in the book Most Common Questions asked by Non-Muslims by Zakir Naik
Definition of Polygamy
1. Polygamy means a system of marriage whereby one person has more than one spouse. Polygamy can be of two types. One is polygyny where a man marries more than one woman, and the other is polyandry, where a woman marries more than one man. In Islam, limited polygyny is permitted; whereas polyandry is completely prohibited.
Now coming to the original question, why is a man allowed to have more than one wife?
2. The Qur’an is the only religious scripture in the world that says,”marry only one”.
The Qur’an is the only religious book, on the face of this earth, that contains the phrase ‘marry only one’. There is no other religious book that instructs men to have only one wife. In none of the other religious scriptures, whether it be the Vedas, the Ramayan, the Mahabharat, the Geeta, the Talmud or the Bible does one find a restriction on the number of wives. According to these scriptures one can marry as many as one wishes. It was only later, that the Hindu priests and the Christian Church restricted the number of wives to one.
Many Hindu religious personalities, according to their scriptures, had multiple wives. King Dashrat, the father of Rama, had more than one wife. Krishna had several wives.
In earlier times, Christian men were permitted as many wives as they wished, since the Bible puts no restriction on the number of wives. It was only a few centuries ago that the Church restricted the number of wives to one.
Polygyny is permitted in Judaism. According to Talmudic law, Abraham had three wives, and Solomon had hundreds of wives. The practice of polygyny continued till Rabbi Gershom ben Yehudah (95% C.E to 1030 C.E) issued an edict against it. The Jewish Sephardic communities living in Muslim countries continued the practice till as late as 1950, until an Act of the Chief Rabbinate of Israel extended the ban on marrying more than one wife.