In order to convince people that the war was needed and have families killing families it could not have been hung on the real reason for the war, which was economics. Some over riding social value had to be used to justify the action.
Exactly, because as bad as Slavery was, it was already on it's death bed around the world and without the Civil War, most likely slavery would have ended about 10 to 20 years later due to dying out from it's own evil. (It died out pretty much around the entire western world around the same 50 year stretch).
But then we wouldn't have been able to place race hatred politics so well for the next 200 years would we have??