There are two great myths of the Civil War.
The Southern myth is that it was a matter of State's right i.e. not allowing a distant part of the country control another. In Europe, it was be like Finland making all the important decisions for Spain.
Sounds good on the surface but Southern Slavarchy would trample all over the right of other states when they wished. Pennsylvania effectively outlawed slavery in 1780, 12 years prior to the constitution but in the eyes of the Slavarchy that meant nothing whereas the fact that slavery predated the constitution by two centuries did. The Fugitive Slave act and the Dredd Scott decision basically imposed slavery on all states in law and only mob violence and the lynching of the odd slave catcher held it back.
But really the purest expression is found in the Constitution of the Confederation itself
Article IV Section 3(3) The Confederate States may acquire new
territory; and Congress shall have power to legislate and provide
governments for the inhabitants of all territory belonging to the
Confederate States, lying without the limits of the several states;
and may permit them, at such times, and in such manner as it may by
law provide, to form states to be admitted into the Confederacy. In
all such territory, the institution of negro slavery as it now exists
in the Confederate States, shall be recognized and protected by
Congress, and by the territorial government: and the inhabitants of
the several Confederate States and Territories, shall have the right
to take to such territory any slaves lawfully held by them in any of
the states or territories of the Confederate states.
No State of the Confederacy could ever choose forbid slavery. So, much for state's rights. This would mutate into the "Myth of the Lost Cause" shortly after the war.
The other myth is that the North fought primarily for abolition. In fact, abolitions were in the main a small radical group of Northern European Protestant cultural supremacist. They were violently anti-Catholic and ferociously in favor of Manifest Destiny and forced relocation of the First American Peoples. Many people at the time, including Lincoln, pointed out that they seemed driven by hatred and envy of the southern class, than out of concern of those enslaved. Abolition seemed almost a side effect.
They arguably triggered the war early and certainly made it more protracted by out and out lying about John Brown and his intentions to foster a mass slave revolt at Harper's Ferry, instead turning into a "Raid." This made it easy for the radical southerners to convince many that the north intended to destroy the south in a race war that would result in the deaths of millions. It triggered the arming of the South, without which the war would have at least been briefer.
They were mistrust prior to the war and as the war advanced, they became actively hated, especially by the soldiery (there were notable exceptions Lawrence Chamberlain for example.) They constantly interfered with military operations being responsible for the loss of the First Battle of Bull Run/Manassas as well as disaster at Battle of Fredericksburg.
Many working class Northern's feared economic competition from freed slaves and the vast majority of Northern whites including Lincoln were dubious that whites and blacks could live together as equals.
In truth, what the North fought for was to preserve the Union of the States. One of the consequences of the Civil War was so seal the Union in blood that later generations forgot that the fear of disunion was a major and ongoing concern right up to the Civil War. The first attempt at secession was in Connecticut during war of 1812 and South Caroline threatened to do so in 1836.
Without an inviolate Union, Democracy was impossible if everyone just stalked off when they didn't get their way. It's telling that the Gettysburg address, considered by most the greatest American political speech, does not mention slavery.
The Emancipation Proclamation set off wide spread dissension in the Union ranks and increased desertions.
But in hindsight, they were right to prioritize maintaining American Democracy even if as Lincoln once said it meant keeping slavery. American in 1860 was the last true Democracy left standing. The counter-reaction to the French Revolution had locked Europe into Aristocracy. Even England was facing a rollback of franchise in its plutocracy. Many in the North viewed the primary threat in the South before the war as a rising hereditary aristocracy instead of slavery.
If the South had won, its possible that succession in America would have been the norm, the country would have flown apart North and South and popular Democracy would have died world wide for half a century or more. Certainly, many elites in Europe were routing for just that outcome.
It was only a good 20-30 years later, that idea that North fought primarily for Abolition took hold. Most likely it did so because at a time of American's rise to the Greatest Nation status, the idea of the entire United States flying apart and popular Democracy dying by then seemed almost silly, so they needed another mythos.
As to why the slaves did not rebel either before or during the war:
The Southern whites were terrified of slave revolts as the majority of them in history simply became slaughters of the slave owners. The history of Haiti and the extermination of the white population there after Napoleon's treachery, haunted them in particular. The South was essentially a police state keeping slaves isolated and unable to coordinate.
During the war, some slaves feared an overkill reaction if they rose up behind the lines. In the past, just the men had been executed after trials. This time they feared extermination. But such fears were very rare. As well, as part of Lincoln's attempt at reconciliation, Union Armies were under orders to prevent uprising.
But perhaps more importantly, the Civil War had an odd sense of propriety running through it. (Although we shouldn't romanticize it.) But, for example, when Sherman marched through Georgia and South Carolina, the southern white males that remained would flee to the hills with their slaves and other chattel while leaving their women folk at home to deal with the Union Army. It's odd war in which one side feels confident enough in the other sides morality that the first side leaves their women unprotected against the other.
That same sense of propriety lead slaves who wished to fight for their freedom to runaway to the Union lines and become uniformed soldiers, instead of murdering their masters or fighting as guerrillas (something still frowned on.)
Telling, in the last two years of the war nearly entire counties lost their military age white male population which was defined very broadly. In many cases, farms and plantations were being run by a handful of white women and dozens and sometimes hundreds of slaves. Yet, virtually none of the women came to harm and in at least one case, a group of slaves lynched another that had tried to sexually assault his white mistress. Ironically, the same sense of propriety prevented the male slaves from revolting when their only targets were women and girls.
- Pre-war slave revolts were prevented by the police state.
- Northern's didn't fight to end slavery as much as to preserve American democracy. (Although, no Democracy, no chance of freedom for slaves.)
- Slaves that fought for their freedom did so formally as part of the Union Army, in the hundreds of thousands.
- Slave revolt during the war was doubly dangerous and would be suppressed by both the Southern and Northern forces
- Every far behind the lines, slaves seemed unwilling to do harm to women, girls, boys and old men. Especially, as it became clear the South would lose.