The reason is that the Southern states and Northern states disagreed on a good bit of ideas. The Confederacy believed that the federal government over America was flawed and they would be better off on their own.
Spread of Slavery:
The south wanted to spread slavery to boost southern economy but the north wanted to spread industry and rid of the old system of slavery. This led to a difference of ideas the Democrats in the south disagreed and wanted to spread while the Republicans in the north wanted to spread industry.
