^I can see that being the reason why more specific mission-ships weren't seen during the Dominion war (kinda like how the USS Titan wasn't recalled during the Borg invasion in the novels), but the Ambassador class was supposed to be the Starfleet's mightiest and was just one generation behind the Galaxy class.
Maybe the Ambassadors were all just assigned to offscreen fleets - probably along with all the Niagra, Chyenne, Intrepid and Sovereign class ships.