I attribute it to, for lack of a canonical answer, a "Star Trek ism" or simply a mistake by the writers. I find often in the Star Trek universe things are done that do not make "real world" sense. I wonder if there is a way that someone could ask the executive producer <PERSON>, or one of the other people involved with the show; such as, <PERSON>, <PERSON>, <PERSON>, <PERSON>, and or <PERSON>. Maybe they could clarify the hows, and whys, and more accurately answer your question.
From my perspective a starship would be designed to have a long life-span, most likely a 100 years or more, would be a logical assumption. For example current naval ships is about 50 years, as with the CVN 65 USS. Enterprise, and aircraft (i.e. B-52, C-130, A-10, F-15) 35+ to nearly 100 years. As with current military craft, there are periodic upgrades, and scheduled refits to keep them current. If I am not mistaken the C-130 is up to J model, for example. With that said, my explanation is "Star Trek VI: The Undiscovered Country" was to be the last movie for the "TOS" crew, so along with Captain <PERSON>, the "USS. Enterprise, NCC 1701-A" was retired as well. A visual cue to the fans "Kirk's Enterprise", is not associated with another captain. To this end the logical assumption is the "USS. Enterprise, NCC 1701-A" was decommissioned because of damage it had sustained; a good reason would be that the space frame, or chassis was damaged making repairs unfeasible, or cost prohibitive. So Starfleet renamed a new "Excelsior Class refit" to be the "USS. Enterprise, NCC 1701-B" as a homage to the previous ship, and crew. That would explain why the ships are so closely spaced. 126
(A side note: I always felt the Excelsior was going to be the next Enterprise, from the first instant I saw it.) Of course they had to tweak it some to give it a different feel, and as a marketing tool a "new ship" that everyone will want to purchase... Rant: ever notice that each series has their own uniforms??? Star Trek Generations was made to be a "Bridge" between the Generations. <PERSON> passing the movie torch to <PERSON>. The "USS. Enterprise, NCC 1701-B" symbolizes the progression toward the “Next Generation”.
The "USS. Enterprise, NCC 1701-C" was made to be a "Bridge" between the "USS. Enterprise, NCC 1701-B" and the "USS. Enterprise, NCC 1701-D". Its intent was to show a design lineage of the ships. It then became a plot device for (TNG: "Yesterday's Enterprise"); a back story was developed for it, but inadvertently as which sometimes happens in Star Trek, it was not thoroughly developed. Or for that matter was omitted from the episode. However I concur with the explanation that the next Enterprise was planned to be the newest starship class, as for the gap in time a logical explanation would be that the “Galaxy Class” development was delayed. Memory Alpha has the class active in the 2350’s, so it may not have been as long as it seems. A good reason for that would be the size, and complexity of the design. If I am not mistaken the “Galaxy Class” was the first class to be able reconnect with the stardrive without the aid of a starbase. So in theory developing the latching mechanism could have taken longer than expected. I hope this helps, this is my first answer; I apologize if my answer is too long, but I felt it necessary in order to be thorough. 126