Because it sounds better for Confederacy supporters to claim the rebellion was over the tariff than over slavery.
Of course slavery was the real issue. But that doesn't say though that tariffs rising from 5% at the start of the country to 40% in the 1800's wasn't more beneficial to some regions and financially harmful to others. The cotton and tobacco economy of the south gained nothing from the protective tariffs. The South had lost the political clout to put in place a funding solution more to their benefit, or to put a stop to the spending on transportation projects that primarily supported the west and northeast.