“Still in 15 states of the Union slavery was illegal.”
During Lincoln’s War there were 16 states in the Union where slavery was legal - if you count West Virginia as a legitimate state, and if you honestly believe the southern states never left the Union.
You were asking why a Constitutional amendment to end slavery was never tried as a means to avoid the rebellion. At that time the rebellion broke out there were 15 slave states. West Virginia was still part of Virginia. Or were you not aware of that?
14 states. Maryland and Missouri outlawed slavery before the war ended.
This was my point. Lincoln's entire justification for starting that war was that the States were still in the Union and were simply in rebellion. He needed this claim to make legal his demands for soldiers and resources to put down the "rebellion" because the constitution gives him the power to suppress rebellion.
Of course this was clearly not a rebellion. It was an effort to attain independence. Lincoln just called it a "rebellion" to unlock the powers he needed to stop it.