"Clearly they were not motivated by protecting plantation owners. No one reason is supportable as "the" reason."
25% of white families had at least one slave.
Whites overwhelmingly believed in slavery in the south, in part because it protected the employment opportunities they whites did have. Of course, they were not willing to share.
Slaves were expensive so I doubt the percentage. Only rich folks could afford them.
The sad fact of the matter is that like most social change, it didn't happen until technology made it possible. The north was in the industrial revolution where factories were the norm. There were no automated farming equipment, it took man hours. That's why even those in the north who opposed slavery on a philosophical level, had them if they needed them, like Washington and Jefferson.
Men like John Adams, who was a lawyer, could survive fine without a slave. So the whole mess would have solved itself instead of going through a horrifically bloody war waged with old battlefield tactics with modern technology. It was one meat grinder after another.
If the south had broken away without incident, they would have come back for economic opportunity. Same with women's sufferage. Technology and economics shapes man more than the moral high ground.