I would assume that it's in tribute to the fact that without the United Earth Ship Enterprise NX-01, neither the Coalition of Planets nor the United Federation of Planets would have been established
Does Trek-lit really have it that all the Enterprises
I mean, we don't know about NX-01 one way or another onscreen, as regards UE Starfleet, and she seems to be retired before there's a UFP Starfleet. Kirk's ships never seemed to carry that nomer, and we saw too little of the -B and -C to tell whether the lack of such mention was evidence of absence or just absence of evidence. Only the -D and -E have been established as ships of flagship status outside lit.