The North Eastern United States was rapidly being industrialized. There were a growing number of cotton mills, manufactering, banking etc.
However, the Southern States, also known as the Bible Belt was mainly dependent on agriculture. The local people were farmers and landowners who relied on the land and their lifestock for an income.
They were able to make large profits by using slave labor which was far cheaper than hiring regular labor.
The eradication of the slave trade was seen by them as an attack on their livlihood.