According to what I've read, what it comes down to is that Diane M. had the misfortune of being in TOS when she was young and very cute. So ... naturally, she had a good time, then. Her experiences of those days coloured her experiences during TNG, where the vibe was very different. She did not feel that she fit in and, in fact, was under the impression that everyone was out for themselves. She was glad to leave and did not want to come back.