you are right that Islam didn’t come up with the order of abhorrence of the practice, but it highlighted the sufferings and did encouraged good behavior with slaves which was otherwise not considered by the society. It is Islam which gave powers to Bilal (RA) to criticise Hazrat Umar (RA) policies openly. There was something in Islamic teachings that attracted slaves and poor towards it. It was not something implied, it was actions of the Prophet and his companions that reshaped the society and its behavior towards slaves. We don’t hear cruelties like Ummaya bin Khalaf and likes did to their slaves by our Muslim salaf.