Here's why:
* Simile: A simile is a figure of speech that uses "like" or "as" to compare two different things. It directly states the comparison.
* Allegory: An allegory is a story, poem, or picture that can be interpreted to reveal a hidden meaning, typically a moral or political one. It often uses symbols to represent abstract ideas.
"Needle in a haystack" doesn't tell a story or use symbols to represent anything deeper. It simply compares finding something small and difficult to find within a large, disorganized space.