I think the likely studio explanation of this is that they wanted more focus on the Galaxy-class and not on the Constitution-refit and thus didn't necessarily want to show a Galaxy next to a Con-re during the airing of TNG, but I don't really understand why the powers-that-be would worry about something petty like that; additionally, there were probably tons of Miranda-class models available for use.
Real-world: TPTB were probably afraid that the casual viewer would confuse any Constitution class ship with Enterprise and think that Kirk was going to show up any second.
Not quite true. TPTB would have gladly re-used the TMP Enterprise model...especially if it meant that they would not have to build a new model. This exact scenario would have happened with Picard's Stargazer if Greg Jein hadn't built his new model in time to be filmed. It also might have happened in "Yesterday's Enterprise" (on Drexler's blog, Okuda talks about how the producers initially didn't want an Ent-C model built, which would have forced them to rewrite the episode so that the ship would have been the Ent-A or Ent-B). Luckily, none of the above happened, and the only time we see the TMP Enterprise in TNG Trek is the destroyed model built for STIII, in BoBW Pt. 2.
The real-world reason why we saw the Miranda class over the Constitution class was because ILM built a Miranda class CGI model along with the Akira, Saber, Steamrunner, and Norway class CGI models for First Contact. When ILM was forced to turn over all its CGI meshes to CBS for remapping and use in DS9, the Miranda was included. Besides the Norway mesh which was accidentally lost, this is why we see an inordinate amount of Akiras, Steamrunners, Sabers and
Mirandas in the DS9 fleet scenes. Conversely, in order to create a CGI version of the Constitution, they would have had to scan the original physical model, something they never did (a CGI model was
built for the director's cut of TMP, but that was after DS9 had finished).
As for an in-universe explanation, I don't think there is a good one. There's nothing inherently special about the Miranda class that makes it any different from its other contemporaries (the Constitution, Constellation, Soyuz and Sydney classes), and yet we see tons of them in the fleet scenes and zero of the other classes. And if ILM hadn't built a CGI model of it, I guarantee we wouldn't have seen it at all in those scenes.
I dont find it shocking or strange at all.. The USAF B-52's have been an active service for almost 60 years now. When they are retired some of them will be almost 100 years old.
In that regard, sure. But the problem here is that lots of other, newer classes were built as well, and none of them were shown in the fleet scenes, while tons of Mirandas were. Would it be realistic to have an airplane fleet composed entirely of B-52s when there are tons of other more advanced jets and fighters?