It was far more a war over tariffs and trade. The South wanted to trade with England and France without paying a federal tariff. They believed the federal government was only to keep out entanglements with foreign powers, not make domestic policy. This making of domestic policy did exist before the war, but it was not until the South attempted to leave the union, did the war begin.
Lincoln had said,, " I will preserve the union if all slaves are set free, or will preserve the union if no slaves are freed". Then, after the Battle at Gettysburg, it was considered a very good political strategy to make the war seem to be more focused of slavery and it was brilliant! It was not so much, the people from Illinois, and Michigan hated slavers, as it was seen to be more relevant to 'free soilers' who did not wish to see salve owners coming into Kansas and Nebraska, as well, slavery was never seen as a good institution by either side, but had, in the South, been such an economic institution for such a long time, they could not reckon any remedy. Abolitionists were a minority voice until recent times.
As it turned out, the war broke the South and made growth in America much more lucrative. Many of wealth in the North used the broken South as a means for business expansion. Until more recent times, (around the time of WWII, the evils of the 'peculiar institution' was seen as a side issue, if much at all.
If one does not examine history written before 1945, then one will not take note of this and believe the North was always more moral and concerned about the slavery issue. Most history books on the war and old South have been revised since this time.