C.E. Evans wrote:
Things change, and Starfleet has always been kind of arbitrary about what naval traditions they observe and others they ignore, IMO.
But that doesn't address the question of why.
Actually, it does, because things go in and out of fashion for whatever reason, and Starfleet is likely no different. If this was a period in which Starfleet became more militaristic in nature, then the choice of uniforms would reflect that nature. Conversely, the next uniform design (the original TNG uniforms), may have reflected a change in Starfleet's priority as an exploration agency.
The cycle could actually have repeated itself with the First Contact
uniforms representing another change in Starfleet's priorities in the post-Wolf 359/Dominion War era.