According to Nichols in her autobiography, Roddenberry wouldn't let her out of her commitment to Star Trek
in order to play a key role on Mannix
. I don't have the book on hand, though, so someone else would have to check the details.
Another book -- I believe it's Inside Star Trek: The Real Story
-- says Nichols was paid so little per episode that she had a "no quote" agreement, meaning that she was paid so little the production company agreed not to publicize the amount. That in mind, I see no reason why she wouldn't want to go on to greener pastures.