Good point. I think heretofore I've mostly assumed that it was related to provisions that a ship would have to carry for the crew; even in a closed system, without replicators I think there would be some loss, and certain things that a ship just couldn't manufacture, i.e. the turkey in "Charlie X."
I've privately assumed that a leap in synthesizer technology took place sometime during Kirk's era, with the possibility of replicator introduction in the 2290s or so given the appearance of something that appeared to be one in Kirk's quarters in TUC, Kitchen notwithstanding. It's still possible that true proper industrial replicators able to make ship-sized components didn't come about until the era in which the Ambassador
was built - and it actually almost works in Excelsior
's favor; if Excelsior
is smaller, then her smaller parts would probably be simpler to replicate than those of an Ambassador
. Gradually, the technology would catch up in time to make a big difference for the Galaxy.