“You” isn’t even historically nominative, but comes from the accusative/dative form of the 2nd person plural (Old English Ä“ow). The old nominative, ye, is now an archaism. So, too, the original 2nd person singular forms, thou, thee and thy.

The loss of thou etc. seems to have been a consequence of the distinction made between polite and familiar forms on the model of French. “You” would’ve been both polite singular and polite plural and, I expect, familiar plural. Consequently, “you” must’ve been generalised to the detriment of the original singular forms.