Why was T'Pol exempted from Starfleet uniform protocol?

Question

Beginning in Season 4 of "Star Trek: Enterprise", science officer T'Pol received a Starfleet commission granting her the rank of Commander. However, afterwards she continued to wear civilian Vulcan clothing. This attire choice made sense when she was a member of the Vulcan High Command (during the first two seasons) and, later, a civilian (third season). As a member of Starfleet, though, she would be subject to the same dress code as other crewmembers.

Out-of-universe, her attire was obviously given the "7 of 9" treatment (attractive actress in tight-fitting bodysuit). I'm curious about the in-universe explanation, though, and wanted to know if it was ever addressed within the show or its spin-off materials.

While in Starfleet, why was T'Pol exempted from Starfleet uniform protocol?

Answer 1

According to the EU novel Star Trek: Enterprise - The Good That Men Do, after she received her commission from Starfleet T'Pol continued to wear a modified Vulcan uniform because she wanted to (it representing a link to her former life as an officer in the Vulcan forces). In turn, Archer was happy to tolerate this because of his respect for her on a personal level and presumably seeing no reason to push the issue.

T’Pol turned and picked up one of Trip’s royal blue uniform jumpsuits. After the Vulcan High Command had cashiered her, Starfleet had granted her a commission. Yet she had never donned their uniform. Perhaps the Vulcan uniform she still wore—a garment that now bore Starfleet commander’s pips—represented an illogical attachment to the past.

After

Trip's death she evidently reconsidered her position.

"And perhaps Trip’s death signified that the time had finally arrived to move past such impulses."

T'pol chose to switch to wearing a Starfleet uniform in his honour, his death representing the point at which she began to fully accept that she was actually in Starfleet and that it was time to stop thinking of herself as an outsider. Note that Archer is vaguely bemused by this decision rather than anything else.

The captain recalled how he’d felt four years ago, that he didn’t want a Vulcan on his ship. Now, he couldn’t imagine Enterprise without T’Pol. His science officer, quiet, competent, and still able to surprise her captain. This morning he stepped out of his ready room and immediately noticed that something was off. Looking towards the science station, Archer saw T’Pol in a Starfleet uniform. Even now he had to suppress a smile. Turning toward the science station, Archer asked, “T’Pol, have you found any ships in the vicinity that might correspond to the warp trail we followed here?”

Answer 2

Ratings. Unfortunately, the decision was not made with an in-universe reason but was in fact for ratings. From this article, Branon Braga was asked why the uniform changed. The quote from the article, below::

During this time, the producers ordered massive changes to T’Pol’s uniform. Why? "Obviously, there's the ratings," co-creator and executive producer Brannon Braga said. "But the primary reason was a creative one. Last season ended with T'Pol leaving the Vulcan high command, so she would no longer wear the same uniform. And, after two years, our leading lady needed a change. She had been in that brown uniform with that little bowl haircut ... and Bob Blackman came up with some dynamite costumes."

Costume designer Robert Blackman gave a different explanation. "The ratings dropped," Blackman explained. "That's the frank, real answer. If you want the show to run seven years, you have to think about demographics." The show was trying to appeal to the key 18-34 male demographic and "there wasn't enough raw sex appeal" according to Blackman.

Blalock was not happy about the costume changes telling SFX Magazine, "You can’t substitute t*** and a** for good storytelling. You can have both, but you can't substitute one for the other, because the audience is not stupid. You can’t just throw in frivolous, uncharacteristic... well, bull and think it's gonna help the ratings!"

Answer 3

T'Pol is a beautiful woman and a more feminine military uniform would not detract from her ability to command. The Vulcan uniform was an unattractive uniform that did not do her justice.