The percentage slave ownership & slave population were strong indicators of Southern background and secessionist sentiment in much of the South. (Don't read a moral judgement into that statement, that's not where I'm trying to go with this.) This was predominantly controlled by geography. Regions of subsistence farming and less fertile soil weren't good at producing traditional slave crops. Border staters and the Upper South were certainly a more mixed lot ancestry wise and the geography had considerable impact on where folks with/without slaves would want to settle.
Rugged regions like the Ozarks, East Tennessee, West Virginia, portions of NW Georgia and Northern Alabama, were more unionist than secessionist. They were areas with fewer slaves because there were fewer good economic opportunities for a favorable return on slave labor.
Think of this as more of an economic agriculture filter than as in morality terms associated with slavery itself.