Actually, that isn't quite true. See the call of Robert Rhett to the other Slaveholding States, as he called them, posted above. There were several outstanding issues. The reason the slavery issue was so important was because the Abolitionist agitation allowed the Industrial Interest (to whom Lincoln had to reach out in order to achieve the nomination in 1860) to develop, in slavery, a wedge issue with which to beat the South as a region.
The Southern States were the leaders of the agrarian interests, the farmers -- the majority -- against the conniving and cartelization of the merchant and banking class, just as in the 1940's Southern Congressional "mossbacks", committee chairmen who wielded great power, were a huge roadblock to the liberals' plans to lead us all into the sunny highlands of State Socialism. And suddenly, the civil rights movement broke out, with liberal NGO apparatchiks appearing in the South as intervenors in school-integration discussions, launching Saul Alinsky-inspired direct political action, bringing lawsuits, and generally stirring the mud, while liberal senators and congressmen rose in the well of the Senate and House to denounce Jim Crow. The South was an obstacle to socialism, but the South was also vulnerable because of Jim Crow. So guess what we got? As Gomer Pyle would say, "Sur-prahz, sur-prahz, sur-prahz!! Gawl Lee!!"
The Yankee Industrial, Merchant, and Banking Interest has been playing this game for 150 years now -- first the Tariff, then Nullification, then Abolition and the Civil War, then the civil rights movement, and now the Confederate-flag controversy. When are our interlocutors going to begin to see the pattern here and realize that they're tools?
It was all about slavery. Even Rhett's own letters, (not his public pronoucements when he was trying to hide slavery as an issue), along with the letters of virtually all the other secess leaders, emphasize slavery over and over again. You don't even get to the tariff as an issue really without slavery.
Oh, by the way, it's interesting that the two leading advocates of the "southern way of life," and slavery, John Calhoun and George Fizhugh, were died in the wool socialists. BOth believed in the labor theory of value. Fizhugh wanted to socialize everyone---he flat called socialism slavery, and said slavery (as in the SOuth) was the best way to implement socialism.