First, not every ship name was important enough to get rechristened onto a new ship every few decades. Thus most ships in Starfleet don't have A, B, C designations.
As far as why that number range, it was discussed pre-Launch and had to do with CBS reserving specific numbers for future series or some-such, so these numbers were implemented by CBS to cause no conflict with anything official they might do at some point in the future.
Ah, that second point does make some sense. However, since there really is no more connection to canon Star Trek anymore, I can't see that as a reason to deny us that option.
As for the first point, I say "Says who?" Just because we haven't seen another ship with a letter designation doesn't mean there ARE none. Absence of evidence is not evidence of absence (also an argument for why there should be COMMODORES in Starfleet, not Rear Admiral Lower Halves).
Once, I was simply called Mojo. Now, I'm forced into a new name, but don't be fooled, I'm the original STO Mojo!