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!"
