Here's why:
* Nominative pronouns are used when the pronoun is the subject of a sentence or clause.
* You is already in the nominative case.
For example:
* You are going to the store. ("You" is the subject of the sentence)
* I know that you are going to the store. ("You" is the subject of the clause)
There isn't a separate form of "you" for the nominative case, as it is already in the nominative case.