Hunter became a star after working with John Wayne in The Searchers in 1956.
With the apex of his career being the lead (Jesus) in 1961s "King of Kings." And according to Solow and Justman's "Inside Star Trek: The Real Story" he sent his wife down to inform the producers he was no longer interested as he was a movie star. He did TV out of neccessity but disliked it. He definately didn't want the stink of TV sci-fi harshing his karma.