(I shall put two so you can pick and choose)
1) Some in the South argued that slavery was actually good for the slaves. Many believed that slaveholders providing these 'lesser beings' with religion, sustenance and shelter was somewhat of an act of kindness toward them. Equally, many in the South believed that slavery was far preferable to the factory work such people would do in the North.
2) Many were worried that without slavery, the economy would crumble. Several elements of trade and production were reliant upon slavery, and so many Southerners argued that slavery couldn't be abolished, in the interest of national prosperity
Not exactly my area of expertise, but I hope that helps x