The setting is very important to this story. Hemingway is writing about man's struggle -- not a specific struggle, but life's struggle in general. Every day of a human's life, he/she struggles with the world, struggles to survive and to be happy and to achieve dreams. Hemingway symbolizes all those various struggles into man's earliest and most primal struggle, the struggle against nature, to control nature and benefit from it. Santiago's dream, very simple and pure, is to catch a fish. He symbolizes the man who has grown old and fallen on bad times but refuses to give up the struggle. He is the human spirit. Manolin represents the young man embarking on his quest to dominate and be successful. When Manolin tells Santiago at the end that they will go out together, readers are left to assume that the dream will be realized. Santiago falls asleep with a positive image, the lions he once saw in Africa, and feels confident and reassured.
If the setting was not so primal or so generalized, the theme would not be so realized. Make Santiago a fisherman living in Florida, and there would be more civilization built up around him and his village. This would taint the conflict with more details from the time period. Set in Cuba, the village can exist within any time, allowing the theme to be broader and apply to a wider audience.