Here's why:
* Oxymorons require two contradictory terms. An oxymoron is a figure of speech where two seemingly opposite or contradictory terms are used together for effect. For example, "deafening silence" or "living dead."
* A single word can't be contradictory to itself. A single word can have multiple meanings, but it can't inherently contain two opposing concepts.
Let me know if you have other word puzzles!