The main reason why classical allegories are not regarded as realistic fiction is because they almost always contain characters and settings that are in no way realistic, unlike most fiction which uses characters that could possible reside on earth.
Step-by-step explanation:
Classical allegories are metaphorical in nature. This means that they contain metaphors, imagery and symbolical messages all throughout the story. Stories are often philosophical and political in nature that could impose morals or enlightenment. Allegories are complex in nature and are often thought-provoking and can be interpreted uniquely by each of its readers, viewers or listeners. On the other hand, realistic fiction is a type of fiction in which the story has elements that can be seen by the audience as something that is currently existing and believable to them. A person, place or animal is known to the audience and the author; and these elements are only used to create a different story that is fictional with a realistic setting.