It was all about slavery. But it would also be a mistake to think it was those who wanted to keep slavery on one side, and those who wanted to abolish slavery on the other. The abolitionist position was still a minority one in the North at the time of the war. The South didn’t just want to preserve slavery, but wanted to expand it westward. Most in the North - including Lincoln himself - were fine with letting the South keep their slaves, but didn’t want it to expand as that would foreclose much of stolen western lands to white yeoman farmers (i.e. the Free Soil movement). IIRC Lincoln offer the Southern states the right to keep slaves in perpetuity so long as they stayed in the Union.