Or major political resistance to the idea of building a deep space fleet? Vulcans weren't hot on that idea in 2151 yet. NX-01 was a proof-of-concept vessel; perhaps all decisions on gearing up production would have to wait for the concept to be proven.
Also, there did exist a UESF before NX-01 was launched. It operated at least a few largish combat starships, and apparently had done so since the first few decades of the 22nd century. In thirty years, one would suppose there'd be enough personnel, enough accumulation of experience, that Starfleet would feel the need to have at least six line ranks, instead of just three. That way, experienced people who still are at working age could be organized into a functional hierarchy, instead of having too many of them agglomerate at Commander level while waiting for ships to be captained.
Granted, though, that we know rather little about the pre-NX-01 Starfleet. Perhaps it only had those three ships seen in "The Expanse"? Alternately, perhaps it had dozens upon dozens, and had already fought a couple of interstellar wars in the minor leagues (say, the one with the Kzinti that Sulu mentioned in TAS). Perhaps it was a major source of pride for Earth, and attracted the best and the brightest - or then it was a humiliating reminder of the fact that Vulcans clamped on everything interstellar, and nobody wanted to serve in this runt of an organization that had no authority over anything else besides a few experimental spacecraft?