^Because that’s not what we saw on screen?
Other than Wolf 359 and the Inquiry cut-and-paste fleet from PIC, the only large Starfleet fleet shots we’ve seen on screen were the DS9 fleets. And they consistently showed only Akiras, Sabers, Steamrunners, Excelsiors, Mirandas, four or five Galaxies, and two or three Defiants. These fleets represented everything Starfleet could throw against the Dominion, so there’s no reason to logically assume that there were any other classes of ships available, as unlikely as that sounds.
Of course, the reality is that this doesn’t make sense. If there were a multitude of old Excelsiors and Mirandas available, then there should logically be a multitude of Ambassadors, New Orleans, Springfields, Challengers, Cheyennes, Freedoms, Niagaras, Nebulas and Olympics available as well, not to mention the 15 or so conjectural classes, all of which were constructed after the Excelsior and Miranda classes.
The only explanation I’ve come up with is that most of those above classes only have a few ships to their name, and their registries are close together. That implies that only a few of them were actually constructed, and probably around the same time per class. Conversely, the registry range for the Excelsior (2XXX-4XXXX) and the Miranda (1XXX-3XXXX) are all over the map, indicating they were constructed over a long period of time, and many more were made.
So when you have 100 Excelsiors in service but only 10 New Orleans, then it makes more sense that we didn’t see any of the latter in the fleet but lots of the former.
It also stands to reason that the proliferation of the Akira, Saber and Steamrunner classes came about after Wolf 359, and were the replacement ships Shelby referred to, despite their low registry numbers. But instead of making 40 to replace the ones lost, Starfleet made 400.
(However, none of this explains why the Miranda class was so special that it lasted for 80 years while its contemporaries, the Constitution, Constellation and Soyuz classes, were retired much, much earlier.)