because the South got a law passed to allow them to go into Northern states and drag escaped slaves (and sometimes free black people) back to the South.
remember this when they try to cast the civil war as being about "states' rights". They wanted the federal government to stomp on the rights of free states. They put in their constitution that no confederate state had the right to be a free state. They tried to use force and violence to annex free states. They didn't give a fuck about states' rights. Anytime a conservative is talking about freedom he's talking about two freedoms in particular:
his freedom to do what he wants
his freedom to use violence to force you to do what he wants
remember this when they try to cast the civil war as being about "states' rights". They wanted the federal government to stomp on the rights of free states. They put in their constitution that no confederate state had the right to be a free state. They tried to use force and violence to annex free states. They didn't give a fuck about states' rights. Anytime a conservative is talking about freedom he's talking about two freedoms in particular:
his freedom to do what he wants
his freedom to use violence to force you to do what he wants