The Badger wrote:
I've speculated elsewhere that as part of the Khittomer Accords in TUC Star Fleet agreed to a strategic reduction in the numbers of heavy cruisers. As each new Excelsior comes into service, an older Constitution is withdrawn to keep numbers within treaty levels. As the Fleet has no replacements for the Miranda planned at this time, they remain in service.
As I say, pure speculation on my part. But it's an explanation.
It is a good explanation, actually.
I'm not a big fan on the idea of the then-vulnerable Klingons dictating how many and what kind of ships the more secure Federation has.
I'm more inclined to think the Khitomer Accords was just an agreement to end hostilities and aggression between nations, but otherwise left their forces intact, with the Constitution-class nearing the end of its design life anyway at the time (I don't think the design entirely went away after Star Trek VI, but no new Constitution-class vessels were built).