Why did southern states secede during the American Civil War?

As with most cases, there were number of reasons.

Economy was perhaps the most important reason and the major component of that was the threat of abolition of slavery. In the south, capital (land) was abundant but people scarce. The opposite was true in the north. The rich landowners of south would have lost big time if the slavery was abolished as the wages would have been driven up by sudden demand for free labor over slave labor.

Other reasons include the cultural difference between the industrialized urban north and the more rural south. It is the kind of similar tension that exists between cities and country sides today. And the role of central government was also important. The central government of the CSA had much less power over the states compared to the Union. Southern states preferred wide autonomy of the states over the federal government where as the Northern states were more willing to submit to the federal system.

There are some starting points for you to consider reading up on.

 

Many Americans today, particularly southerners, deny that the reason for the civil war was the issue of slavery. However, South Carolina(the first state to secede) cited "increasing hostility on the part of the non-slaveholding States to the Institution of Slavery" as the primary reason in their declaration of secession.

Also if you need a reason for why the south might have still been struggling even that long after the war, look into Sherman's March to the Sea which was a Union military campaign where the Union army basically destroyed everything in its path and disrupted the southern economy

In addition another reason the southern economy still hadn't recovered fully is that it was based entirely on labor that was free during slavery and still basically free during the share cropping system

 

Isn't this obvious? The South was very different culturally from the North at the time. And then there was the especially obvious racial tension and concern over the legality of slavery.