Why does StatesRights keep denying that the south seceded because of slavery?
Because there were other reasons besides Slavery. That was A reason, but not THE ONLY reason.
If slavery were the only issue, the Southern States would have (peacefully) re-joined the USA when Lincoln endorsed the "Corwin Amendment"
"No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State."
The Emancipation Proclamation did not free any slaves in Kentucky, Missouri, Maryland, Delaware, and West Virginia, nor any southern territory already under Union control.