Jesus had to die on the cross because the Lord is so pure that He cannot look at sin which means He could not look upon us unless a blood sacrifice was made on our behalf. In the Old Testament we find that when people sinned they had to go to the priests with a firstborn animal from their flock and give a blood sacrifice to cleanse and purify them. People had to do this every time that they sinned in order to get back in God's good graces. Jesus was the ultimate blood sacrifice for sins. Jesus sacrificed His life so His blood would cover our sins because God could not look upon us, no more animal sacrifice was needed to cover people's sins.