Here's why:
* Frame narratives have a story within a story. A main story acts as a frame for a secondary, more central story. Examples include:
* *The Canterbury Tales* (pilgrims tell stories)
* *The Arabian Nights* (Scheherazade tells stories)
* *Heart of Darkness* (Marlow tells the story of Kurtz)
* The Wizard of Oz has one main storyline: Dorothy's journey to Oz and back home. There are subplots (like the Scarecrow's quest for a brain), but they are all integrated into the main story, not distinct stories within a larger frame.
Let me know if you'd like more examples of frame narratives!