Well, she left the ship, but they found ways to keep her in most of the remaining episodes after that. Also, I realized that I could probably go through the list and name another half dozen or so people, but the ones I named were all off the top of my head. Nahn was another who was regular cast for a while, left, then they brought her back for one or two episodes.
In a way, I think it's actually kinda cool and different. They wanted to do something really different with this show, and in a more "realistic" setting (given that this is science fiction), crew members would indeed come and go, and die from time to time if the adventures have any real stakes. It makes more sense than going seven years and the same five or so people are always there and always have been, protected by plot armor.