I think a number of factors contributed to the American Civil War, some which date clear back to the founding of the country.
For one, some of the founding fathers argued strongly that slavery be abolished when the new country was formed, however, this idea was radical and unpopular. Another radical and unpopular idea was one of a strong federal government that superceded a state's rights. Some felt that states should have the power of "Nullification." Or, the right to "ignore federal laws with which they disagreed." This right was never approved by the federal government. So, right from almost the very beginning of the country, the die was being cast for conflict.
Next, the socioeconomic factors must be looked at.
In the North things were becoming industrialized. People worked in factories. There was need of human labor but in a closed in setting, and where one received wages for labor. The profit for the factory owner was higher, yet he did not have the cost of maintaining his employee, only needing to pay him or her wages. Services too would grow up around the factories.
The South was still decidedly agrarian. But, with Eli Whitney's invention of the cotton gin, the South began having a boom. Only their boom was in cotton. Before the invention of the cotton gin, slavery had been falling into disfavor. Now however, farmers began switching their crops to cotton. It was easy to grow and grew practically anywhere. With the gin, a farmer could increase his harvest more than he'd ever thought possible. He just needed workers to glean it. Cheap immigrant labor was there, but not willing to do the back breaking labor for the wages offered. Slave labor was the recourse and slaving became a thriving business again.
The states were trying to resolve the issue of what new lands could or would or should be free or slave back in the early 1800s. The Missouri Compromise was reached in 1820 regarding the Lousiana Purchase area stating that at 36 degrees and 30 minutes, except for Missouri that new land would prohibit Slavery.
The matter of land issues came up again after the Mexican War. California was said to be a free state, but Utah and New Mexico were given "Popular Sovereignty." The could decide for themselves through popular vote. This was also to be the way Kansas and Nebraska were to decide. Pro slavery "vote stuffers" who lived along the Kansas border made their way into Kansas to try and rig the vote. The times turned bloody as huge clashes between pro and anti slavery factions caused many deaths in Kansas. Still, the state voted to be a free state and Nebraska soon followed
In 1950, the Fugitive Slave Act was passed. It made it Illegal for Federal employees to not turn in a slave or face fines. And made it illegal to hide a slave, that a slave was still a slave no matter if he or she was in a free state or a slave state. This act, along with the publication of Uncle Tom's Cabin cause many antislavery proponents to increase their call for the end to slavery.
Tempers were running hot, Harper's Ferry was raided. Lincoln was elected and immediately, seven states seceded. South Carolina leading the way. Lincoln wasn't even president yet.
But, they were afraid, afraid Lincoln would put an end to slavery and thus put an end to their way of life. Without slavery there could be no cotton, without cotton there could be no South as they saw it. It would be an end to their economic structure. To their lifestyle.
So, was it about slavery? Yes, but not only slavery. Was it about state's rights? Yes, but not merely state's rights.