Here's why:
* Interrogative: "Why" is used to ask questions.
* Adverb: It modifies verbs, adjectives, or other adverbs. For example:
* "Why are you late?" (modifies the verb "are")
* "Why is this book so boring?" (modifies the adjective "boring")
Let's look at why it doesn't fit into the other categories you mentioned:
* Intensifiers: Intensifiers make adjectives or adverbs stronger. Examples include "very," "extremely," "really." "Why" doesn't have this function.
* Negatives: Negatives negate a statement. Examples include "not," "never," "no." "Why" is not a negative word.
* Nouns acting as adverbs: Nouns can sometimes function as adverbs, but this is usually in a specific context and with a different meaning than "why." For example, "I walked the whole way," where "whole" is a noun acting as an adverb modifying "way." "Why" doesn't have this kind of usage.
Let me know if you'd like to explore other word categories!