Um, let’s go with Slavery!
Problem with that theory is that it does not fit the facts. Lincoln stated in his first innaugaral address that he would support the Corwin Amendment which would have made slavery effectively permanent. He also said as late as August of 1862 that if he could save the Union without freeing any slaves, he would do it.
Add to that the fact that the Northern states kept slavery six months longer than the Southern states, and it becomes hard to argue that Slavery had much to do with why the Union decided to invade the South and get 750,000 people killed.
Nope, it was about control of the economic commerce of the South, and nothing else.