Why Polygamy is allowed in Islam?

Polygamy means a system of marriage whereby one person has more than one spouse. Polygamy can be of two types. One is polygamy where a man marries more than one woman, and the other is polyandry, where a woman marries more than one man. In Islam, limited polygamy is acceptable and polyandry is wholly prohibited.
Qur'an is the only religious book on the face of the earth that says 'marry only one'. The context of this phrase is the following verse from Surah Nisa of the Glorious Qur'an: 'Marry woman of your choice in twos' threes' or fours' but if ye fear that ye shall not be able to deal justly, (with them), then only one' [Al-Qur'an 4:3]

This means that Islam only allow you to have more than one wife if you are capable and you are able to justify. Moreover you are instructed to marry needy or helpless women to give her support and help. If you are not able to keep her safe and happy then you are not allowed for polygamy, only limited polygamy is allowed in Islam.