I think of the Connie refit as a stopgap measure, intended only to keep the fleet barely modern until Reliants and especially Excelsiors achieved critical mass in the fleet, and ultimately not that good a ship (re)design, even for their time.^Which would explain why they went back to fusion based impulse engines in later models.
The way I see it, the refit Enterprise was something of a prototype, whereas the E-A is more of a regular production model.
Otherwise, it's baffling why Reliants and Excelsiors are still in service in the 24thC when Constitutions aren't.
"She can still outrun us and outgun us." [emphasis mine]