If you look at a Physiographic map of the USA you'll notice that every state in the Confederacy had substantial regions in the southern coastal plain. That's a geographic region which is suited toward growing crops (cotton in particular) on large plantations. If you look at the distribution of slaves, you can see it largely follows the coastal plain. But note that Tennessee has only a small portion of coastal plain and a lower slave population. Despite being geographically somewhat in the middle, it's in many ways "on the edge".

You can see the pattern even more clearly in West Virginia, which broke off entirely, is located in a different physiographic province, and had very few slaves.

My great great grandfather fought for Tennessee on the Union side. I wonder if this is why.