The Miranda might simply be a more powerful ship than the Constitution. At least in firepower. The refit Constitution model has no visable aft torpedo tubes. it does have good phaser coverage. but then the Miranda likely has all arcs covered as well as the Constitutions with less phasers due to the more compact hull.
The Ambassadors may have been a limited production run, like the Galaxy class was originally going to be. In Kirk's day there were a little more than a dozen Constitution-class ships like Enterprise
. In Picard's day there were suppose to be six Galaxy-class ships with six ready for completion if needed. The Ambassador-class may have been a similar project with a dozen of them built.
As oppose to the Miranda-class and Excelsior-class which do not seem to have been very limited in their runs. Or some of the "First Comtact" based ships which fill out the fleet with the Miranda and Excelsior hulls during the Dominion war, along with the enlarged number of Galaxy-class ships and some Nebula-class ships. The only missing ship from the newer group is the Norway-class.