This page purports to have the answer.
It says in part:
"Had the official split occurred 90 years earlier, the boundary may have been the crest of the Blue Ridge. Had the Civil War occurred in 1850, then West Virginia may have included what is now the Ninth Congressional District. Once the Virginia and Tennessee Railroad was built, connecting Southwest Virginia with Tidewater ports, subsistence agriculture was replaced with a cash economy based on tobacco. As part of the cultural change, slavery became more common in the region - and of course the counties went into debt to help finance the railroad. The boundary commission that recommended what counties should be included in the new state purposefully excluded those counties with a heavy debt load."