You are not recognizing the initial issue, just a symptom. The initial issue from the get-go was States Rights verses a strong Federal Government.
The battle between the States and the Federal Government was the true issue. Slavery was only ONE symptom. Others included Interstate Commerce, etc.
The Civil War was not a one issue (slavery) fight.
The idea that this country is a union of STATES, and the states are SOVEREIGN save for the very limited realms in which the constitution allows the federal government to operate, is anathema to leftists. To leftists, that idea must be stamped out.