Why? Shouldn't it have kept the same reg number, and our well known Defiant be the Defiant-A?
If the NX-74205 hadn't been the prototype vessel for the Defiant class, you would probably be right. I believe it's the only instance of an on-screen named vessel reappearing as the namesake for a ship class.
Just speculation here, but it might be to do with the enterprises being the flagships. Being an iconic line of ships they kept the registration. Most of the other ships didn't really get the kind of fame the Enterprise did, with the exception of the defiant. The constitution defiant never really accomplished much, so Sisko's defiant probably didn't share the registry because there was no real sentiment to it. Not sure what the Sao Paolo's registry is, it could be the same as the defiants since that did get a lot of fame. It could also be different since it had another ship name and was renamed defiant to replace the lost one. Most of the other Enterprises were built to be Enterprises, either after the old one was obsolete or destroyed. The Sao Paolo was built to be another defiant class but they gave it to sisko as a replacement.
No facts here, just personal thought.
Harden up Princess
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Seriously? The Enterprise is the Federation Flag, always. Like Air Force 1 will always have that designation, even after the actual plane is replaced with a newer model.
Bad example. Air Force One has had numerous different registry numbers over the years. Not to mention any Air Force craft with the President onboard becomes Air Force One for the duration of the flight.
Its usually just the writers not being that nitpicky about things and often for simplicity of the audience who are not always die hard fans.
For example, they knew in DS9 that the romulan and klingons were in the beta quadrant, yet everyone only talked about defending the alpha quadrant from the dominion. despite most trek fans knowing what is what, they did not want to confuse people by adding an extra quadrant name in, so everyone ending up coming from the alpha quadrant. nice and simple.
same with the defiant probably. calling it the Defiant A will have confused people who did not see that one particular TOS episode. The Enterprise in TNG needed it because not having the registry number change would have confused casual watchers as to which ship this was. They specifically needed to know its a later model.
It leaves an inconsistency that annoys hardcore fans, but makes life simpler for everyone else.