My understanding is that Hunter's decision came from career advise he received from his wife, she wanted her husband to push for movies roles, and she didn't take seriously this little science fiction series out of a declining studio.
Well, we don't know that for sure. All we know is that his wife is the one who came to see the screening of "The Cage" and told Roddenberry that her husband wasn't interested in returning for a second pilot. We don't know whether she was the instigator or just the messenger.
That was the story as reported in the behind-the-scenes books from the 90s IIRC. Hunter's wife pushed him towards movies and away from the Trek part. Hunter supposedly made a perfunctory offer to return, but at with a salary demand that all-but guaranteed that he would not be asked back.
More interesting to me was the whole kerfulffle over Number One and Spock, both of whom NBC wanted GONE post haste. GR could only save one, and almost had to chose the alien for story reasons.