Well, back to our studies. Where were we?
I don’t know why we use the instrumental here. Maybe because the rest of the cases make lesser sense.
It cannot be the nominative because it denotes the subject alone.
It cannot be the genitive because it denotes smb or smth from which smth derives or comes from or is taken from.
It cannot be the dative because it denotes smb or smth to which the action is addressed.
It cannot be accusative as it was explained above.
It cannot be prepositional for there is no preposition.
Sadly, my answer is “I don’t know”. Maybe you should forget what I wrote above.