The thing is the Excelsior class was brought online 40 years after the first Connie had been launched. The Excelsior was basically the next generation of multi mission ship and could do everything the Connie could if not better. So why waste time and resources keeping a small class of starships updated and in service when numerous Excelsior and Miranda hulls had been commisioned and formed the bulk of the fleet.
Actually i think there were fairly few Excelsiors by the time the connie was retired, but with peace with the Klingons just around the corner Starfleet would have changed focus from combat heavy ships to more science/exploration oriented ships, like the Oberth and Miranda, building only few heavy ships.
Just look at the TNG era: How many ships have we seen total of the Galaxy Class in TNG and DS9 (pre Dom War)? The Galaxy Prototype (we never seen it, but it must have been there right?), The Enterprise, The Oddyse, The Yamato and the Venture... It was not until the Dominion Wars we saw more combat able ships, suchs as Galaxies, Akiras and the Defiant of cause.
Personally i think the it had not been until around the lost era (ENT C era) that Excelsiors went into mass production, because by then the design would have been refined and perfected enough to support the resources put in to production of thoose ships.
That would also explain why there are so many Mirandas around, because they, and the constallation class, would have made up the bulk of the fleet between TS:V and TNG: Cheap to build, Easy to modify, and only a small crew is required.