Because they didn't trust him. Because they thought slavery would be safer if they had their own country.
The problem was Lincoln wanted the Southern states to pay the Union a 40% tariff on their exports. The South considered this outrageous and refused to pay. By the time hostilities broke out in 1861, the South was paying up to, and perhaps exceeding, 70% of the nations taxes. Before the war, the South was very prosperous and productive. And Washington, D.C., kept raising the taxes and tariffs on them. You know, the way Washington, D.C., keeps raising the taxes on prosperous American citizens today.
Tariffs applied to the whole country not one region. Southerners who didn't like the tariff increase could have stayed in Congress and blocked it. The initial increase was from 17% overall (ad valorem), or 21% on dutiable items to about 26% overall or 36% on dutiable items. The tariff went up further later to pay for the war. And no, Southerners weren't paying 70% or more of the nation's taxes in 1860.
Not only did blacks not riot against the whites of the South, many black men volunteered to fight alongside their white friends and neighbors in the Confederate army. Unlike the blacks in the North, who were conscripted by Lincoln and forced to fight in segregated units, thousands of blacks in the South fought of their own free will in a fully-integrated Southern army. I bet your history book never told you about that.
Have you shame? Or no brain? Try reading more than your own lying history book, Chuck.
And correct me if I'm wrong but the tariff on exports was zero percent, not the 40% that Chuck claims.
He does go too far in his claims.