I've always been told that the only reason West Virginia left the wider state of Virginia was because the latter chose to join the Confederacy, but several counties seceded from Virginia so that they could rejoin the Union.
So, if that was the case, why wasn't Virginia reunified after the Union won the Civil War?
A year ago I and u/freedmanspatrol were in a previous discussion on the somewhat complex creation of WV here. Essentially, an alternative government of VA was created in Wheeling, and since secession was not recognized, the Confederate government in Richmond was simply treated as a center of rebellion. That Wheeling government, with a few different conventions, eventually split the state into two states. So, it was the recognized government of VA that did it- that's why eventually it passed Congressional scrutiny and was accepted by Lincoln.
There was a request in 1866 by the Virginia legislature to re-unify: the WV legislature replied that "the people of this State are unalterably opposed to reunion...will not entertain any proposition to that end" *. This was followed by attempt by VA to reclaim two counties in the Eastern Panhandle ( Jefferson and Berkeley) that went to court in 1870. Those counties were culturally much like the Shenandoah Valley and mostly Secessionist. They did not send delegates to the first convention in Wheeling, therefore, having no wish to join. The leadership in Wheeling knew that those two counties had railroad junctions crucial to the new state's economy, however, and conveniently, when there was a final vote for delegates to the new state, those two counties did send delegates- while they were under Union occupation. In the 1870 court case, VA claimed that this had influenced the voting ( which it likely had) but the court ruled against VA.
If there was no attempt to stick the states back together after 1866, it must be admitted that it was also became rather moot, as ex-Confederates (like Daniel Bedinger Lucas) shifted their political aspirations from Richmond to Charleston and in the Constitutional Convention of 1872 pretty quickly gained control of the state and held it for decades. In giving it a government quite in sympathy with the Lost Cause and segregationist, they were not quite able to convert the state back into a Southern one, though, never quite managing to restrict voting rights and government offices to whites, never quite doing away with free public schools .
*Federal Writer's Project, West Virginia: A Guide to the Mountain State ( 1941)