Why was slavery not definitively prohibited in Islam, even though it is considered a deprivation of human liberty and enslavement?

To answer this question, please refer to fatwas number: 35861, 32223, and 32160. Among the books concerned with refuting misconceptions are: "Fi Zilal al-Qur'an" by Sayyid Qutb, and "Shubuhat Hawla al-Islam" by Muhammad Qutb.