What's written in the religion is to free slaves and take care of them. Banning slavery would have caused chaos and make people flee away from Islam. It's similar to alcohol. Alcohol is haram but it was not haram in the beginning. That doesn't mean that God changed his mind. It means that in order to bring change about it takes a process. Imagine banning alcohol in any US country tomorrow? Will it have a positive effect? No but can we agree that the world would be better with less alcohol because of drunk driving, drunk sexual assaults etc etc? Sure so you take steps to reduce it and eventually phase it out which is what Islam did with alcohol and which is what it did with slavery.