It could be that the Ambassador Class just did not perform as well as expected in the field. The Miranda Class to me seemed to be like the Jeep - a good basic platform which was easy to build and maintain and could be adapted to a number of uses. We have seen different external equipment changes and the bridges have been upgraded to current tech when we saw them. The Excelsior is probably another good reliable design which has been upgraded over time- I think the Enterprise/Lakota variant was an upgrade to the physical exterior that fit a more specialized purpose. The Enterprise was supposed to be a top of the line ship so it had extra impulse engines (I really wish though they had kept them as shuttle bays as shown on the sets Master Systems Display) and revised secondary hull/warp nacelles. The Lokota was also intended to be a top of the line ship under the control of Admiral Leyton so having the extra bells and whistles is fitting.
I remember there have been a number of aircraft designs which seemed great on paper and in simulations, but when actually built and flown turned out to be disappointing. They were OK and could be tweaked, but they were not as good as hoped so were not built in large numbers. I think the Ambassador Class is similar- it was intended to be the next big thing, to replace the Excelsior Class, but simply did not work out as well in the field so the Excelsiors stayed in abundance filling those roles.
I remember there have been a number of aircraft designs which seemed great on paper and in simulations, but when actually built and flown turned out to be disappointing. They were OK and could be tweaked, but they were not as good as hoped so were not built in large numbers. I think the Ambassador Class is similar- it was intended to be the next big thing, to replace the Excelsior Class, but simply did not work out as well in the field so the Excelsiors stayed in abundance filling those roles.