Below is a map from the 1860 census showing the slave population of the Southern States (Darker = more dense population of enslaved people). You can basically draw higher union support in the counties with less ties to the cause of slavery. It's not a coincidence that the free state of Jones has the lowest rate of slavery in Mississippi or that the Free State of Winston has the lowest rate of slavery in Alabama, or that Winn County Louisiana with the lowest rate of slavery in the state was the only county in the state to not support secession. You can basically draw West Virginia there from the demographics of the enslaved.
Governor Isham Harris was largely the mouthpiece for the secessionist movement in Tennessee with his base in Central and Western TN. He publicly sent out a list of proposed Amendments that Tennessee would not secede/rebel if they were taken up (along with 21 complaints Tennessee had). EVERY proposed Amendment was about protecting and expanding the institution of race-based chattel slavery (as well as 19 of the 21 complaints). They were the usuals... Protect slavery in slave states. Protect slavery in slave territories, ensure its expansion would be protected, put more teeth in the Fugitive Slave Act. Stop abolition movements. If you've read the Crittendon Compromise, you get the point. This cause and list didn't motivate people in the areas of the South where enslaving people was less prominent.
In Knoxville "King Harris" or Benedict Arnold Harris as local leaders and papers would often call him was not well received for his pro-secession/pro-slavery movement. In order to try and secure a pro-secession vote, Harris had been using the state militia to overrun federal installations (much Like other upper south states). The Federal Arsenal in Nashville was taken, Ft Brown... Ft Wright. Basically, every military depot or armory they would come across, they'd send an armed group in to take it. BEFORE a vote on secession. In Eastern TN they saw these raids on federal bases for what they were... a way to push the TN vote to secession. Those pre-emptive raids didn't sit well with a group that wasn't really wanting secession in the first place.
In the east, you had Andrew Johnson as a political leader representing their interests who was a staunch unionist/Jacksonian Democrat, believing nothing good would come of secession for Tennessee. One of the few Democrats to win out there. Largely Eastern Tennessee rather than send their vote to the extremes (Southern Democrats for example) chose the more moderate political party which had been the Whigs that campaigned there for the most part.
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It was Geographic in part. You had mountainous terrain that somewhat isolated it from the rest of the state, particularly the plantation heavy areas in the west. It was economic, more small-scale farming, subsistence agriculture/industry and less reliance on slavery-based plantation economics.
And you had where the people came from. During the early 19th century, Eastern TN wasn't being populated by plantation owners and slavers so much as those from the Northeast, and in fact some that had moved away from slavery, not to expand it west.
Religion played a huge part here too. By the time of the secession crisis the major protestant denominations had their splits over the morality of slavery. But one of the more interesting bits of history was in Eastern TN, you had some of the strongest abolitionist/anti-slavery religious leaders in the South. Rev. Isaac Anderson (founder of Maryville College), Rev Samuel McPheeters, Rev. James Lyon, Rev Robert PL Jackson, and Rev William Brownlow (Would become governor in 1865). The moral idea that slavery was evil was something that had more of a foothold there than arguably anywhere else in the South. Multiple pro-slavery/pro-secession arguments through the south calling out the evils of abolitionists were based on the example of Eastern Tennessee of what the rest of the South may end up being if that movement wasn't stopped.
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