Money. The Southern states were pumping 700 million through the Northern economy each year, and they were producing between 72% and 78% of all the taxes used to run the government.
Secession would cost the North millions as the South took away much of their European trade, and stopped paying the tariffs that kept the US government running.
One thing I didn't put in my earlier post is that the pro-Confederate National Volunteers group were the ones who tried to kill Lincoln in the Baltimore Plot (before Lincoln was even inaugurated). I wouldn't be surprised if that impacted Lincoln's decision to follow his predecessor's (Dim Buchannan) in keeping U.S. troops at Ft Moultrie/Sumter and trying to send them supplies.