Look, it's quite simple:

The north had slave states. There were riots in New York where northerners killed black people. The only reason the north didn't have as big a slave industry was because of the weather. The northerners would have rebelled if they thought that they were all dying to free the slaves. No, they were fighting to "preserve the union". And likewise, southerners were not fighting to keep their slaves. As mentioned, the vast majority of them did not own slaves.

The Emancipation Proclamation was a great publicity stunt by Lincoln to try to make the war about slavery. Why? Because he didn't want Europe to jump in on the southern side, which it was about to do.

But what did the Emancipation Proclamation actually do? Nothing! If Lincoln had really wanted to free slaves why didn't he free the Northern slaves? Why "free" the southern ones he had no control over?

It was a great publicity stunt by Lincoln, and ever after it has made a good simplistic "white knight black knight" tale for the history books and public schools- but it just isn't correct.

The war was not about slavery, but about economics...as most wars are.