Why was there no explicit verse revealed in the Qur’an prohibiting the buying and selling of human beings and freeing all slaves, even though Islam is a religion that honors and dignifies humanity?

Islam came when slavery was widespread, so it restricted it to those captured in legitimate jihad. Then, it strived to emancipate slaves and opened avenues for manumission, such as expiations for oaths, Dhihar (a form of divorce), and killing. It also allowed a slave to be freed if their master was freed, and for a female slave to become free upon giving birth to her master's child. Furthermore, it opened the doors for mukātabah (a contract where a slave pays a sum to their master for freedom) and tadbīr (a master's declaration that a slave will be free upon the master's death). It also mandated the completion of the emancipation of a partially freed slave, all aiming to gradually eliminate slavery. Some scholars have pointed to the non-existence of slaves from Allah's saying, "And whoever does not find [a slave]" when mentioning emancipation in the expiation for Dhihar, which indicates the non-existence of slaves due to the opened avenues for emancipation. Islam honored humans, and if it permitted slavery, it was due to its prevalence, and it was gradually eliminated.