That's not so. Slavery was a big issue before the war and may have been a minor issue towards secession but was not the main cause.
I'll start with Georgia's Declaration of Secession:
http://www.yale.edu/lawweb/avalon/csa/geosec.htm
Slavery is mentioned directly as the main reason for secession in
the first sentence of the document. That gives slavery a position of prominence on that document as the Preamble and the 1st Amendment has in the Constitution and the Bill of Rights. They mentioned it first because it was first and foremost on their minds.
And they
kept talking about it.
The words "slave", "slavery", or variation thereof appear over 30 times in that document, by my quick count.
Economic reasons are discussed, but all
within the context of justifying slavery.
Mississippi's Declaration of Secession:
http://www.yale.edu/lawweb/avalon/csa/missec.htm
They were so incensed about tariffs and economics that they don't even mention any of that stuff at all.
Nope. They just launch into a lengthy discussion of slavery starting with the
second sentence of that document.
"Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world."
Again, there is probably a reason why they mention slavery in the second sentence and then devote the entire rest of the document to the defense of that institution...
Texas' Declaration of Secession:
http://www.yale.edu/lawweb/avalon/csa/texsec.htm
Have to hand it to the Texans. They waited until the fourth sentence in their document to mention slavery.
"She was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery-- the servitude of the African to the white race within her limits-- a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time.
Slavery seemed to be first on their minds at the time. And they certainly weren't talking about it as if it were going away anytime soon.
South Carolina's:
http://www.yale.edu/lawweb/avalon/csa/scarsec.htm
Easier to read than the Texas document and certainly more scholarly and respectable...but there it is again in the first sentence of the document. That's what was on their minds.
E. S. Darghan's speech to the Alabama secession convention:
http://www.sonofthesouth.net/leefoundation/Alabama_secession_Speech.htm
Slavery is mentioned in the third sentence. In fact, his speech touches on no other subject besides slavery.
I could go on, but nobody was really talking about import tariffs. When and if they did, it was a distant second to the major issue on their minds. Slavery.
What purpose does it serve to pretend otherwise? These are their own words, not those of "historians" living and breathing today.