Shatner & Nimoy were singed for every episode in season one. Whitney & Kelly were main characters but were not signed for every episode. I think they found it easy to write for McCoy (who I think is in all but one season one episode, whereas Whitney appeared in only 8 out of 13 episodes, some of which were little more than cameos). They painted themselves into a corner with Rand because they made the unrequited passion too up front from the start and they kept having to pedal backwards to let Kirk flirt with other women instead of building from a low point as is common today. She was written out of Dagger of the Mind because it would have shown their relationship being bumped in a new direction that was too full on and eventually she was written out altogether for a variety of reasons but a couple of which were that she was too expensive to employ if they couldn't think of how to use her in a way that couldn't be done by a Yeoman of the week, and she was a millstone around Kirk's neck because if he shagged another woman he was effectively cheating on her, although it's as likely that behind the scenes reasons could have played a bigger role. I would certainly have preferred to see her stick around for one or two episodes per season. McCoy on the other hand was such a joy to write and so successful that he became a season two regular.