^How do we know it wasn't?
If it was, it would have been galloping around the cosmos with a different name as well as a different crew. U.S.S. Enterprise, NCC-1701-B launched about a year after the events on Khitomer.
Or maybe not. The interval between Star Treks VI and VII is unknown. The decommissioning of the Enterprise
-A and launch of the Enterprise
-B could very well have been in the same year, only months apart.