Why did Islam premote slavery ?

Why did Islam reform slavery and not outlaw it? I hear slaves where treated good but a slave is a slave which is a possession. The Islamic slave trade was the deadliest of all slave trades. Over 75% of slaves were women. And I want to know why did Muslims slaved people from non Muslim nations when at war. The Quran calls it riches of war. But most slaves where civilian women and not soldiers! Even children of slaves where slaves! In the Quran it is premitted for the master to have sex with his slave woman. According to a Hadith even Ali the son in law of Mohammed ( pbuh) had sex with a slave!