Background: Eugene Louis Vidal was born in the cadet hospital of the U.S. Military Academy at West Point, New York, the only child of Eugene Luther Vidal (1895-1969) and Nina S. Gore (1903-1978). Vidal was born at the West Point cadet hospital because his first lieutenant father was the first aeronautics instructor of the military academy. The middle name, Louis, was a mistake on the part of his father, "who could not remember, for certain, whether his own name was Eugene Louis or Eugene Luther". In the memoir Palimpsest (1995), Vidal said, "My birth certificate says 'Eugene Louis Vidal': this was changed to Eugene Luther Vidal Jr.; then Gore was added at my christening [in 1939]; then, at fourteen, I got rid of the first two names."
Context: In 1956, Metro-Goldwyn-Mayer hired Gore Vidal as a screenplay writer with a four-year employment contract. In 1958, the director William Wyler required a script doctor to rewrite the screenplay for Ben-Hur (1959), originally written by Karl Tunberg. As one of several script doctors assigned to the project, Vidal rewrote portions of the script to resolve ambiguities of character motivation, specifically to clarify the enmity between the Jewish protagonist, Judah Ben-Hur and the Roman antagonist, Messala, who had been close boyhood friends. In exchange for rewriting the Ben-Hur screenplay, on location in Italy, Vidal negotiated the early termination (at the two-year mark) of his four-year contract with MGM.  Thirty-six years later, in the documentary film The Celluloid Closet (1995), Vidal explained that Messala's failed attempt at resuming their homosexual, boyhood relationship motivated the ostensibly political enmity between Ben-Hur (Charlton Heston) and Messala (Stephen Boyd), that Boyd was aware of the homosexual subtext to the scene and that the director, the producer and the screenplay writer agreed to keep Heston ignorant of the subtext, lest he refuse to play the scene. In turn, on learning of that script-doctor explanation, Charlton Heston said that Gore Vidal had contributed little to the script of Ben Hur. Despite Vidal's script-doctor resolution of the character's motivations, the Screen Writers Guild assigned formal screenwriter-credit to Karl Tunberg, in accordance with the WGA screenwriting credit system, which favored the "original author" of a screenplay, rather than the writer of the filmed screenplay.  Two plays, The Best Man: A Play about Politics (1960, made into a film in 1964) and Visit to a Small Planet (1955) were theatre and movie successes; Vidal occasionally returned to the movie business, and wrote historically accurate teleplays and screenplays about subjects important to him. Two such movies are the cowboy movie Billy the Kid (1989), about William H. Bonney a gunman in the Lincoln County War (1878), occurred in the New Mexico territory and later an outlaw in the Western frontier of the U.S. and the Roman Empire movie Caligula (1979), from which Vidal had his screenwriter credit removed, because the producer, Bob Guccione, the director, Tinto Brass and the leading actor, Malcolm McDowell, rewrote the script and added extra sex and violence to increase the commercial success of a movie based upon the life of the Roman Emperor Caligula (AD 12-41), which is the fourth biography in The Twelve Caesars (AD 121), by Suetonius.
Question: why did he have his credit removed?

Answer:
the producer, Bob Guccione, the director, Tinto Brass and the leading actor, Malcolm McDowell, rewrote the script