According to Herb Solow:And, per Justman and Solow (if I recall), the production was happy to move on from Hunter because his wife was such a nuisance. She wanted to be rid of Star Trek, and Star Trek wanted to be rid of her.
HERB: We had an option on Jeff Hunter for a series, but not for another pilot... the lights came up, Jeff Hunter's wife [actress Sandy Bartlett] gave us our answer: "This is not the kind of show Jeff wants to do, and besides, it wouldn't be good for his career. Jeff Hunter is a movie star."
--Inside Star Trek (hardcover), p. 63
--Inside Star Trek (hardcover), p. 63
I don't recall her being mentioned elsewhere. If she was, someone find the quote?