But the differential between the regular Excelsior and its upgrade to the secondary hull was minimal at best, and perhaps not deemed necessary with advancement of technology for all Excelsior class ships. So, some like the ENT-B and Lakota had them, but others did not (even though they sported latest internal upgrades which kept the ships on par with modern ships?).
The Constitution class apparently went through massive design alterations some time prior to TOS which led to Federation wide adoption/change in design aesthetic (or at least sections of it).
It could be argued not all of the ships and stations in the fleet changed... perhaps it was a limited change which was witnessed during TOS era.
I wonder if SNW or another Trek series will actively start changing things to the TOS look too but keep other designs intact.