Abe Lincoln only made slavery illegal because he wanted an indirect way to provoke a war with the south.
The south had long since been planning to abandon national government and leave governmental power up to the states - this would have meant that Washington DC would have been in control of fuck all.
So, since the south's economy relied heavily on slavery, Abe knew that making it illegal would start the Civil War early and prevent the south from building up enough power to successfully secede from the union.
Fact