Till you look at the facts.
3/4ths of the Trade money represented by this tariff graph was created in the South. Southern Independence meant the direct loss of 3/4ths of all European Trade, and the idling of the bulk of the Northern Shipping/banking/warehousing/insurance industry, not to mention eventual direct competition with their textile and steel industries.
An independent South was a dire threat to the North Eastern financial interests. This is why Lincoln was willing to let them keep slavery, but was never going to allow them to have free trade with Europe.
It was about money. Specifically the money that would be lost to the Shadow Government of New York Industrialists.