Re: Constitution, Miranda, and Constelation Class
They needed to build another Excelsior model because they couldn't use the original one, as it had the Ent-B parts permanently stuck to it.
Interestingly, during pre-production for Generations, the producers were going to have an all-new model built for the Ent-B, of a different class. If that would have happened, then the original model could have been used for "Flashback," and we might have seen more ships of the Ent-B's new class later in DS9. But that's not what happened.
Though ironically the reason for the extra bits on the Enterprise-B was to avoid cutting into the original model, so it could be damaged and subsequently restored to its original condition.
Obviously that didn't quite go to plan!
She bought her first new car and you hit her with a drunk driver. What, is that supposed to be funny?