Here's why:
* Pronouns are words that stand in for nouns. They can be used as the subject or object of a sentence.
* Nouns are words that name people, places, things, or ideas.
* Verbs are words that describe actions or states of being.
In the sentence "Why did you go to the store?", "why" is asking for a reason, which is a noun.
It acts as the subject of the sentence:
* "Why" (the reason) "did you go to the store?"
However, "why" can also be used as an interrogative adverb, which is a type of word that asks a question.