Based on letters I’ve read of Union soldiers, had they been told that the war was to free the slaves, they never would have enlisted. If Lincoln had declared emancipation early on, enlistment would have been much lower, and I believe the draft would have had to be enacted long before it was in 1863.
I think you’re missing a key point. Just because slaves were “emancipated” by decree on paper, that did not make them free in reality.
Think about it.