When West Virginia seceded from Virginia, how come the southwestern part of what is now the Commonwealth of Virginia didn't join West Virginia when they are geographically and culturally a lot more like West Virginia then the rest of Virginia?
Actually, West Virginia includes parts of south-west Virginia, basically all the counties south of Kanawha County in WV. At one time the new state would have taken in the Shenandoah valley also. The state of West Virginia was made arbitrarily and not on county loyalty, as you can see in this map of the West Virginia counties that voted for the Confederacy.
The Wheeling constitutional convention argued for weeks over what counties to include. As it was, they bit off more than they could chew because when the war was over and the people of West Virginia got their right to vote back they trashed the Wheeling constitution for one a little more like the old Virginia constitution.
At the constitutional convention Mr. Van Winkle explained his reasons for including Confederate counties in the new state.
For our purposes, there are three areas: Northwest West Virginia, Southeast West Virginia, and Southwest Virginia. That is if you divide the state using a "line" running southwest to northeast.
This map shows that only "northwest" West Virginia was anti-secession. Southeast West Virginia was pro-secession. Meaning that "West Virginia" was basically "too large" based on anti- and pro-secession sentiments.
Yes, Southwest Virginia was "like" Southeast West Virginia. One could make the argument that they belong in the same state. But if that argument had prevailed, they would both be in Virginia and not West Virginia.