Comparing to the other options, some sort of slavery was prevalent before Muslim conquests. The Purdah was a gift of Muslim invasions, and same applies to Child marries which came into practice to avoid the new danger of lifting Hindu girls by Muslims as an holy act of Jihad, Sati & Jauhar also got established as the wives of the invaded used to kill themselves before their husbands went on the last battle.

Slavery became more prevalent because it was a fashion and a status symbol among the Muslims at that time. As we read Aladdin had 84000 slaves and Firuz Tughlaq had 2 lakhs, all of them were maintained at the state cost.