Possibly because Kirk Douglas bought the film rights to the original book. Douglas wanted to play the title role, but by the time the film was green-lit he was to old to play the role himself. His son Michael produced the film with Jack Nicholson in the starring role. (Few people are aware that when Michael Douglas won the Best Actor award for "Wall Street" that it was his second Oscar. His first was for producing "Cuckoo".) I cannot prove it, but I believe "Amos" was written as a vehicle for Kirk Douglas to finally play an older version of McMurphy.
