And in fact, Grace Lee Whitney, who was originally intended as the female lead of TOS, the Miss Kitty to Kirk's Marshall Dillon, was the person who recommended that they switch to the skirts. Here's what she had to say on the subject in Allan Asherman's Star Trek Interview Book:
GLW: I was in pants. I rebelled, because to me it should have been a big belt with a short skirt, boots, and long legs. That's what I saw. And when I said that to Bill Theiss, he had the same image. Bill designed the shorts with the skirt flap over the top: the famous uniform.
Asherman: Whose idea were the pants?
GLW: I don't know. They were in the pilots, before I got there. When we put the legs into the format I think that helped sell the series.
Asherman: What do you think of the costume you wore in the series?
GLW: I thought it was sensational. It stopped traffic, and being an egocentric actress, the more attention I got, the better.