In OTL the Confederacy didn't seem to care about defensible, or nearness to Union borders. Commercially important did matter, though, the Tredegar Iron Works was one reason Richmond became the Confederate capitol.
Without Virginia, the largest population Confederate cities would be New Orleans, Charleston, Mobile, Memphis, and Savannah. I’m not sure which had the most manufacturing.
And has been pointed out, simple inertia might keep it in Montgomery.