There's not a huge amount of logic, no pun intended. Ann Mulhall, an astrobiologist of equal rank to McCoy, should obviously have played a key role in the Immunity Syndrome and if not her then her replacement. And wouldn't have loved to see Helen Noel and Marta go one on one in Whom Gods Destroy? Continuity just wasn't a major priority.
Also they would have had to pay both ladies for a return appearance and probably the writers of their original stories, especially if said original writers were not scripting the stories in question and the suits didn't like paying out!
JB