Because the human nature is to have higher power, and the human nature algorithm can go wrong, resulting in a belief that human sacrifice to higher power is necessary for more life for self. Jesus is the only sacrifice you cannot improve upon. You start out with libations, milk for the gods, food for the gods. That doesn't work, you give the gods blood. Animal, then some other human, then your own child, then your first born males, then the king's first born son…
God sent His only Son as a final sacrifice. You cannot one up this sacrifice. It's final. No need to give God what He first gave you. That message is really good news. Everyone should share it, because humanity can forget, collectively. We eat his flesh and drink his blood in wine and bread to REMEMBER what we don't ever want to return to.