Slavery was destroyed because many Northerners, specifically the ones following the Free Labor ideology and the abolitionists who saw slavery as immoral, wanted an end to it. In the antebellum period, it was impossible for the North to end slavery on a national level because of the heavy presence of Southerners in Congress. So, near the end of the Civil War, the 13th Amendment which banned slavery, was easily passed because since the Southern Democrats left the Union, they no longer had a say in the matter.

Post-war, The South did attempt to create a system as close to slavery as possible with the Black Codes, but Radical Reconstruction did away with that. Granted, freedmen were still treated poorly by white supremacist Southerners, but it wasn't de facto slavery.