Slavery was rampant in the Arabian Peninsula before the advent of Islam. Moreover; it was never denied in Christianity or in Judaism. It was Islam that laid heavy restrictions on it and enacted laws to free slaves and even implemented moral ethics to treat slaves.

Islam reduced the sources of slavery

The sources of slaves werenumerous at the time of the prophet, among these were:

• Prisoners of war( men, women and children)
•  Banditry and highway robbery, where all members of caravans were taken in as slaves by their captors
•  By a person selling him/herself into slavery or selling one of their children
• Through kidnapping
• Through loans and usury (interest)
• Prisoners of war

Islam only permitted slavery from Holy Wars that were launched in defense of the Islamic state because in this case if slaves were freed, there was the threat that they would have returned to launch war against the Muslims. It is noteworthy that Islam kept them until they were ransomed or given in exchange for freeing Muslim prisoners.

Islam put a long-term plan to free slaves

Unlike Christianity and Judaism, that were empathic on taking slaves, Islam set codes to free them. These were holy ordinances that were to be applied on a large scale. Prophet Muhammad (SWS) as well as his companion, Abu Bakr both bought a large number of slaves and freed them.

Abul Ala Mawdudi says

"The result of this policy was that by the time the period of the Rightly-Guided Caliphs was reached, all the old slaves of Arabia were liberated. The Prophet alone liberated as many as 63 slaves. The number of slaves freed by 'Aishah was 67, 'Abbas liberated 70, 'Abd Allah ibn 'Umar liberated one thousand  and 'Abd al-Rahman purchased thirty thousand and set them free. Similarly other Companions of the Prophet liberated a large number of slaves, the details of which are given in the traditions and books of history of that period. Thus the problem of the slaves of Arabia was solved in a short period of thirty or forty years."

Why didn't Islam abolish slavery at one go?

The social and economic conditions in the Arabian Peninsula were highly connected with slavery. Had Islam abolished slavery all at once, many social and economic problems would have arisen. The slaves themselves wouldn't have had a source of livelihood and also many economic activities would have failed. Islam would have had people who were weak in faith, if their slaves whom they depended on were freed. There was also the possibility that civil wars would have broken out.