The answer is: the South’s economy depended on it.
The movement of abolition, historically and culturally the most important cause of the American Civil War went through many historical phases and turning points that more or less helped advance the cause. However, it is well recorded that the South, being most dependent on slave labor —most notably in plantations and other forms of production—, was reticent to support the cause of the abolition of slavery since it would undercut their finances and personal privileges, both of economic and cultural nature.