Because the north was wanting to force abolition on them but the North would have done nothing but talk if the South had not tried to secede.Hence it wasn’t slavery it was secession that caused it to escalate to war.
Lincoln and the Republicans knew that there was nothing they could do about slavery where it existed. Their aim was to halt the expansion.
.Hence it wasnt slavery it was secession that caused it to escalate to war.
But the reason for secession was what the South saw as the threat to the expansion of slavery so we're back to square one.