My guess is because they were originally intended purely to stop russian anti-tank rifle rounds, which couldn't really penetrate the side armour of the panther or tiger. The shaped charge part only was discovered much later, fully evident by the move to the lighter, easier to produce wire mesh on the late Panzer 4s. So that can excuse why they didn't original design schurzen for them. Why they didn't later add them, I have no clue.