My understanding (which is limited) is that the issue of slavery was brought into the mix by Lincoln, who was basically looking for foreign allies in a conflict which was already brewing. The US was one of the last places where Caucasions were enslaving Africans, and by the time of the Civil War, the rest of the world was largley opposed to the practice.. Lincoln knew that if he made the war seem to be about the abolition of slavery, he could build a coalition to crush the Confederate Army, if necessary.
Add to all that the fact that the winners write the history books, and you get "slave states" and "free states."
That said, you can't say that the Civil War was or was not about slavery without being wrong. Some fought against a federal government, some fought for slavery, some fought against slavery, some fought for a federal government... Some probably didn't know what they were fighting for, but gave it hell anyway.