Slavery has never been abolished. It was simply made illegal in most places under most circumstances. By any definition of slavery there are more people in bondage today than at any time in the past.

Even taking your claim of abolition at face value, the only reason chattel slavery was "abolished" in the US was because its abolition became a way of ending a particularly bloody war by taking away one sides motive for fighting. Before that (i.e. before a majority of the citizenry saw abolition as benefitting themselves) there was not enough popular support for abolition.