“Lost cause” theories are considered the actual revisionism by the vast majority of historians, due to the many pieces of strong evidence supporting slavery being the major issue driving the war.
Let’s start with the articles of secession. Ctrl-F on these and you’ll find “slave” mentioned more than 80 times. https://www.battlefields.org/learn/primary-sources/declarati...
There were the journals of the actual soldiers fighting the war, who frequently mentioned how slavery was their motivation for fighting the war.
There was the intense focus on slavery leading up to the war, like a slave state being admitted for every free state. There was the Caning of Summers where a man was nearly beaten to death on the senate floor over a very rude speech he gave over slavery https://en.m.wikipedia.org/wiki/Caning_of_Charles_Sumner
There were the northerners coming into slave states and inciting slave rebellions https://en.m.wikipedia.org/wiki/John_Brown_(abolitionist)
In general, the idea that slavery wasn’t a motivator of the war was mostly pitched to Britain who was mostly abolitionist at the time, but overwhelmingly it was mostly pitched after the war had already concluded. Before and during the war the confederates were quite clear their motivation for waging the war was primarily slavery. They were very open about this, which is why historians tend to be doubtful about a theory which seemingly was only ever popularized to make the confederates save face. Historians aren’t keen to go “oh these people who mentioned slavery over and over when they were seceding didn’t care about slavery that was just a prank and I know how they really felt, checkmate Lincolnites!”
Nobody doubts that economics weren’t a major factor in the war, but slavery was discussed far far far more often than tariffs. Tariffs are mostly brought up nowadays because they’re more palatable, not because of the strong evidence the average southerner or southern leader was fighting for tariffs.
>Lincoln did nothing to free the slaves when the war started
True. Lincoln’s initial goal was to keep the union together, whereas the confederates were fighting for slavery. Keep in mind the Civil war was by far the deadliest war in US history, I believe more Americans died in that war than every subsequent war combined. Ending slavery is a noble goal, so is preventing the 620k deaths that happened.
It was only later on, largely due to the aforementioned British pressure that he gave his emancipation proclamation largely as a strategic move to prevent Europe from entering the war since after the proclamation their entrance into the war would be seen as supportive of slavery. However, Lincoln did want to end slavery eventually and many in the union fought for the sake of abolition. Note how a civil war was started soon after Lincoln was elected with the secessionists repeatedly mentioning slavery, his support of abolition was no secret.