Why was the militery part of starfleet not using warp five drives if produce more power.

[Jason 27]

First and must importnat remember it protype means that warp five engine. NX Enterpise did not test the top speed until they had to. UFP want more testing time. Talk about give next generation of cargo ships warp five drive partly back this claim.

 

Secound only advance the warp five drive offer was speed it did not generation more power then warp 4 drive.  This back fact that in thoery NX Enterpise could gone up to warp 6.5 by some minor changes to the warp drive itself.  Only other piece of evindece that that Star Trek Twlight were at least one Reptilian ship had been destory off screen. Fact decide send two the ship attack Intrepid. That one Xindi starship destory on screen.  Other last fact Xindi board party only 4 soldiers. If starfleet officer not injured the team get wipe out.  Since fire broke out between three engineer crew members and  took cover able hold at bay until MACO finish off the job. Things could turn out the other way. Even evidence Intrepid class starship more power warship then NX class.

 

Last possible is more simple one engine it is is expensive to build lot cheaper longer starship warp four engine.

 This reason I came up with like here some others reasons though.

[Khas 12,158]

There's also the reason that the Vulcans were deliberately holding Earth back.

[Jason 27]

Once the Human build warp five engine not stop the human give every ship fleet warp five engine come power generation.  Vuclan nothing do with stop the Starfleet doing it. Fairness cargo ships equement warp 3 and more powerful warp drives. Possible even warp four drive some cases. Starfleet militery like use all warp 4 drives as will.