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 As each new Excelsior comes into service, an older Constitution is withdrawn to keep numbers within treaty levels.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.