I remember an article long ago saying that Bujold simply walked off the set after a few days--and nobody went after her.
I do think Berman and company where sorta looking for a "female Capain Picard" when they approached Bujold for the part--even renaming the role Nicole Janeway at Bujold's suggestion--but it simply wasn't meant to be. IMO, Bujold just couldn't get into the role: the technobabble, the way Trek was filmed, as well as perhaps the whole science-fiction aspect--it was literally too alien for her, and she just couldn't take it any more.
Now, of course, we can't really think of anyone else other than Kate Mulgrew as Janeway. She took the part, made it her own, and did a great job with it if I can say so.
Still...the "what if?" part of me will always wonder how the role could have turned out with two other actresses that were in contention for the original role of Captain Elizabeth Janeway:
-Lindsay Wagner (the original Bionic Woman)
-Susan Gibney (TNG's Dr. Leah Brahams/DS9's Captain Erika Benteen)
Free your mind, and the rest will follow.