Well we know it wasn't slavery. In the South the rich invested in slaves, and the rich controlled most of the economy. In the North the economy was dominated by agriculture, and was well along into the industrial revolution.

To put the economies in the starkest of differences, 90% of all the capital in the United States, prior to the Civil War, was located in the North. The North employed 1000% more factory workers, in factories that were magnitudes more profitable. Steel production, railroads, engines, chemicals, all this was produced in the North in almost their entirety.

The South produced only three things of note: cotton, slavery, and traitors.

Interesting. Would you say the South was an important market for the industrialized North?

Would you like to guess whether businessmen in the North invested in the slave trade and grew rich thanks to it?

And what do you make of the fact that the North abolished slavery only after decades of using slaves, making it plausible that they never could have succeeded as colonies without slaves in the first place?

You can't say the northern states were not benefiting from exploitative conditions or even slavery in the south. And all of white America continued to profit from denying non-whites equal rights in one way or another.

The Confederates had probably hoped the North would not want to go to war over the Union thing.

And also: Even though slavery is not an effective means of production (though it probably was, at some points for some people), it's still wrong.