I'm a native of SWVA and always thought this was curious. While the rest of Virginia was full of large estates and had a large number of slaves, my understanding is the western part of the state (including modern WV and the southwest of VA) was economically and racially distinct - having primarily small-plot farmers and far fewer slaves.
In my experience the Appalachian/Blue Ridge mountains are a bit more rugged in WV than VA, was this part of it? What caused the yeoman farmers of the southwest to throw in their lot with the confederacy?
Southern and Eastern West Virginia actually tended to have a larger number of slaves and high support for the Confederacy in general and a large amount of Virginians, Monroe county for instance had around 1,000 slaves amounting to about 10-11% of the population. In regards to SW VA as a whole we are talking about a rather large area with varying degrees of slave holding and support of the Confederacy. For instance Franklin county was about 30% enslaved, opposed secession but ultimately largely supported the CSA once war came. Washington county was about 15% enslaved, students from Emory&Henry( the dominant regional college) supported both the Union and the Confederacy. The college was the scene of a brutal massacre by local guerrillas ( loosely allied with the CSA) who killed black soldiers and a local man who had sided with the Union who were recuperating on the campus-turned-hospital. The other point I would make is that the Union won several early victories in what would become West Virginia, giving them control over large portions of the NW and allowing Unionists to dominate the referendums on creating a separate state. Had the decision included those supporting the Confederacy in all likelihood West Virginia would be notably smaller.