Promotions come slowly when the military numbers are static. Only rapid expansion, heavy turn over, and/or heavy loses cause rapid rank advancement. Since it took the NX era Star Fleet 3 years to complete one starship, the NX-02, without any other space docks building other new starships in sight of Space Dock 1, production of new vessels for Star Fleet must have been slow during the early NX era.
You would think that Star Fleet would have a crash starship production program, crash space dock production, crash starship crew training program, after the Zendi attack on Earth. But there did not seem to be a war time type of training and production rush after the Zendi attack on Earth.
Shortage of people willing to serve in Star Fleet? Shortage of people that meet Star Fleet standards for a rapid expansion of Star Fleet?