Only because there never was a really high-quality miniature of it to film for stock footage/closeups, and the one that was built was too late for stock footage. (a LOT of the Excelsior class footage seen throughout TNG was filmed prior to season 1 airing--they pretty much filmed it once, then it got used for ST VI)
The Ambassador is little-seen for production reasons, not "because it's rare". Sure it's gotta be less common than the Excelsior, but it should be a heck of a lot more common than the Galaxy, Sovereign, and Defiant combined.