There were a variety of reason for the southern states doing what they did.
Obviously, the institution of slavery was a strong motivator in that it would have changed the cost structure to cotton and sugar.
Also, making the (former) slaves equal to poor southern whites would have upset the power structure such that poor whites would have to compete with blacks for the work that was available. This would have changed the electoral prospects for the wealthy whites in the south as well.
The economics of the northern bankers and textile industries would have made land acquisition more feasible, but there is still the need for much unskilled (stoop) labor, that had not been addressed in the planting and harvest of cotton and sugar cane.
To a degree it was economic, but also political, and social as the society would have been reordered without slavery.
The fear of social equality of black people was a very big factor in secession. The majority of people in the South owned no slaves. There was no middle class to speak of in Southern society; there were a small minority of very rich plantation owners and their families, a much larger number of poor white subsistence farmers who basically survived by growing enough food to feed themselves and their families, and slaves. The plantation owners certainly did not want abolition obviously, but mostly for economic reasons - they were wealthy but lacked liquidity or investment capital. Almost all of their wealth was tied up in land and slaves. Ending slavery would have cost them a large portion of that wealth.
The poor white farmers who were the majority of Southerners also opposed abolition, but economics meant almost nothing to them. Freeing the slaves would not have either harmed or helped the poor subsistence farmers making up most of society. They were fearful of black equality. The wealthy plantation owners were harmed by abolition, but they still remained at the top of society. It was this class of poor whites who were most supportive of the Jim Crow laws and Black Codes that were developed after Reconstruction. These prevented true equality for black people and allowed the poor white people yo maintain their position in society.