I know why I believe it was, but I would like to compare notes.
Why was the slavery of the pre-Civil War Southern US immoral?
Ironically enough, it was the religious people who opposed slavery as immoral, and the science book enlightened, who made the argument it wasn't. Essentially all the abolitionist movement was coordinated by Quaker and Unitarian ministers. Yeah, those pesky God believers, who are the source of all our problems in the world these days. While the prevailing "scientific" view of the day was, the African slave was not even a homo sapien. Thank Darwin for this! Because of his theories of evolution, many people simply didn't believe black people were people, they were a step below actual humans. Yes, it's shocking today, we can't imagine such a time, but that was the case in 1860s America.
You can try to have a conversation today about slavery and the Civil War, and people are just so removed from what we used to believe, they can't wrap their minds around it. They assume that we've always considered black people human beings, and we just didn't find it wrong to discriminate against them. We discriminate against horses and dogs everyday, they don't get to vote or have a political representation. But that's acceptable because...well, they aren't humans, right? The same was true for blacks back then, people simply didn't consider them humans, people, citizens. For all intents and purposes, they were akin to livestock. Does anyone think it's wrong to 'enslave' cows? I mean, aside from a few PETA nuts?
Now even though we don't consider animals people, we are still upset with abuse of animals whenever it happens, and so was the case with abuse of the slaves. The majority of abolitionists weren't opposed to slavery because it was wrong to enslave black people, that's a misconception rooted in modern-day thought. They were opposed to the unethical treatment of living creatures, much like people are opposed to dog fighting. Very few people of that time, thought of black slaves as human people on the same level as white human people.
Even Lincoln himself, stated: "I do not believe the negro will ever hold the same station in society as the white." Before the Civil War, Lincoln had actively pursued a plan to repatriate freed slaves (and other free blacks) to a parcel of land the US purchased in Central America, for this purpose. The first black people to be invited to the White House, were invited by Lincoln. They met with the President for about 15 minutes, where they were not allowed to speak. Lincoln informed them that he wanted these black leaders to promote his idea and lead hordes of black followers off to this new land to live free, far away from white American society. Hundreds actually went, and subsequently died.
When the southern states began seceding, Lincoln was actively working with Congress to pass legislation which would have kept slavery legal until 1911. By his own words, the Civil War was not fought over slavery.
"If I could save the Union without freeing any slaves, I would do it, and if I could save it by freeing all the slaves, I would do it, and if I could do it by freeing some and leaving others alone, I would also do that."
The victors write the history books, so we are taught the CW was fought over slavery and Lincoln freed the slaves. Generally, liberal teachers like to juxtapose the plight of the abolitionist movement, with the Civil Rights movement of the 1960s. As if the prevailing mindsets were the same. It was a completely different time, and the abolitionists of 1860s had more in common with modern-day animal rights activists than anything approaching Civil Rights.