When compared to other Enterprise classes such as the Constitution, Excelsior, and Galaxy, we’ve seen a surprisingly small number of Ambassador class ships, both in terms of unique vessels and on-screen appearances. What might account for the relative dearth of these ships?
The production reason would likely be that they had only built that one physical model, and it was used for that one-off, had to be blown up, and was never digitized. The other Enterprises got a lot more screen time probably because they had high quality models they hung onto and reused.
I honestly can’t think of a good in-universe reason outside of them being maybe predominantly used for diplomatic missions more or less exclusively. We also don’t really explore the gap between TMP era and TNG. I think it’s a pity, because it’s such a gorgeous ship. It’s one of my favorite designs.