What do you suppose would have happened if they had pointed out how much money was going to suddenly stop flowing through Northern hands? As some of the articles I posted to you said, "WE WERE DIVIDED AND CONFUSED UNTIL OUR POCKETS WERE TOUCHED." New York Times March 30, 1861".
They did not do so because they were trying to sneak their way out, while not making it apparent that they were going to be making away with a lot of money that would normally go in Northern pockets.
Would you tell someone you are about to rob them, or would you take the money and abscond before they realized what had happened?
What do you suppose would have happened to Confederate efforts to gain recognition from Great Britain if Robert Barnwell Rhett (Author of the document you linked) had pointed out that slavery was the reason for rebellion?
I find this to be a much more plausible explanation why there are so many documents and statements by Southerners themselves addressing the need to protect slavery against the threat of a Republican administration, and so precious few that you can post stating purely economic reasons having to do with tariffs. Even he addresses his document to the other slaveholding states, not the other economically deprived states.