Orson Welles actually wrote the script
Orson Welles was trying to get this script produced back in the 80s. There is a famous story about Welles being turned down by Steven Spielberg. Welles even pandered a bit by offering to include Amy Irving (Spielberg's girlfriend at that time) in the film. Later Spielberg paid $50,000 the remaining prop sled (Rosebud) from the Welles film Citizen Kane. Welles commented bitterly that Spielberg would pay $50,000 for that old prop but not for his original script (Cradle Will Rock).
Robbin's seems to have been given full credit for the script due to some technicality in the Writers Guild rules for assigning credit. Does anyone know more about this?