If the South was primarily or essentially more about the preservation of slavery than anything else then why, when offered the chance by the Lincoln administration to have the US Constitution amended to include a clear affirmation of the inviolability of slavery as the cost of peace and reunification, did the South not jump on it?
As Major General John B. Gordon so aptly stated:
As for the South, it is enough to say that perhaps eighty [percent] of her armies were neither slave-holders, nor had the remotest interest in the institution. No other proof, however, is needed than the undeniable fact that at any period of the war from its beginning to near its close the South could have saved slavery by simply laying down its arms and returning to the Union.
The answer is simple and obvious. They wanted their liberty and independence more than slavery. For them the union, though asserted via a noble compact, had failed. They desired to establish a government more conducive in their own estimation, and by their own consent, to the preservation of their liberties and way of life.