Your forefathers ate manna in the desert and yet they died. But here is the bread that comes down from heaven which a man may eat and not die. I am the living bread that came down from heaven
Miraculous it might have been the manna of Sinai was just another temporary expedient. Just as the blood of bulls and goats could not take away sin, so manna could not give eternal life. Jesus is the bread of life and only his death on the cross would suffice.