Definitely agree that the Ambassador-class should have been more prominently featured after it was introduced in "Yesterday's Enterprise" - certainly should have made appearances during the Dominion War (or more, anyway - not sure I recall it ever appearing in any of the fleet shots). Probably its absence was due in part to the studio model being relatively small (at the time only broadcast TV resolution was necessary) and no high-resolution CGI model was ever made, making it unsuitable for the sort of close-in beauty shots as the Enterprise-D and -E got.
The Ambassador also suffered from a kind of "morphing" design, intended to be an intermediate step between the established Excelsior-class and Galaxy-class, rather than being designed independently as an advanced (for early 24th century) Starfleet explorer. Excelsior was such a popular design because it looked like the design team had taken the original Constitution and redesigned it from scratch to make it more advanced. Sovereign, Defiant, Akira, Intrepid and Vesta are successful for similar reasons, each looks to be designed for a specific missions with appropriate features and innovations. Ambassador had to be "devolved" from Galaxy, and was a lesser design because of it.