Also, as we well know, the later appearances of the model featured many alterations, such as redone ramscoops and lifeboat stations - but also changes that would be rather massive in in-universe terms, namely nudging the positions of the saucer and the nacelles vs. the secondary hull.
Perhaps the original design was outright faulty, requiring these expensive changes to work properly? Starfleet could have been planning on series production (at least on the silver-bullet scale of the later Galaxies
), but decided to jump a generation of giant ships, perhaps first going for the stopgap Niagara
in small numbers and then trying out the Galaxy
We could similarly argue that the Sovereign
design was faulty at first, requiring the structural changes from ST:FC to NEM, and reducing the numbers built or the pace of production.
Of course, the changes in both cases could be minor fiddling or "upgrades" rather than absolutely vital patches. But it does seem that the E-E was almost immediately subjected to a refit. Perhaps the same happened to the first Ambassadors
, save for NCC-1701-C, or after NCC-1701-C's demise.