For most of the series, she did not wear her uniform on duty like other members of the crew. However, after Captain Edward Jellico ordered her to wear her uniform in Chain of Command, Part I, she kept wearing it.
Has it ever been explained why that is in-universe?
I'm aware that there is an explanation out-of-universe but that's not what I am looking for.
My best guess is that, as she explained in Thine Own Self, the events of Disaster made her think about increasing in rank. When Captain Jellico ordered to wear her uniform, it might have made her realize that, if she wanted to be a commander, she might have to act (i.e. dress) like one. However, I cannot find anything confirming or denying that.