Here's why:
* Abstract nouns represent concepts, ideas, qualities, or states of being. Examples include *love*, *freedom*, *justice*, *happiness*.
* "Star-Spangled Banner" refers to a specific, tangible thing: the national anthem of the United States. It's a song, a piece of music with words.
Therefore, "Star-Spangled Banner" is a proper noun, specifically a compound noun, referring to a particular entity.