Then why do so many of the articles of secession explicitly mention slavery rather than taxation? Those were their chance, like in the Declaration of Independence, to express their highest ideals. Some, like Florida and Louisiana, essentially said the union with the United States was dissolved without giving reasons. On the other hand Mississippi said "Our position is thoroughly identified with the institution of slavery - the greatest material interest of the world. Its labor supplies the product, which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin."
The taxation reason appears to be mainly post war revisionism. Preservation of slavery was the reason why the southern states left in 1861.
I would say that slavery severely aggravated the relationship between North and South but was not the immediate cause of the outbreak of war.
After all, how many Iowa, Michigan, and Ohio farm boys - not to mention Irish kids from the cities - were willing to march South to free the slaves? Save the Union, yes; but free the slaves, no.