“Bujold was the first choice of the producers of Star Trek: Voyager to play Captain Nicole Janeway. She quit after a day and a half of shooting, with the public reason being she was unaccustomed to the hectic pace of television filming. (See: Bujold as Captain Nicole Janeway) Other rumored reasons included dissatisfaction with her performance on the part of the producers and dissatisfaction with the character on the part of Bujold. As Rick Berman politely put it in the October 8-14, 1994 issue of TV Guide: “It was immediately obvious it was not a good fit.” The producers subsequently hired TV veteran Kate Mulgrew, and changed the captain’s first name from Nicole to Kathryn on Mulgrew’s advice”

Something tells me Berman is the real reason.