I think this is because the Southern states were heavily reliant on slave labor because of the agricultural economy flourishing there and the South supported slavery while the North did not. The slaves provided cheap labor for the plantations in the South and they believed that their region's economy would be destroyed without slaves. The North had more urban cities which did not require much slave labor and had an overall moral opposition to slavery. With their opposing views and constant tension, the South eventually started to secede from the Union. Hope this helped!
Fugitive Slave Law brought the issue home to anti-slavery citizens in the North, as it made them and their institutions responsible for enforcing slavery
