It's the same reason you can't buy a kidney in most countries. As soon as you make it possible to buy life, you can create a system that exploits the poor.

E.G: I was watching an episode on kidney trade (Netflix TV show The Traffickers) and a man was duped into selling his kidney. He expected more but only received less than $300. There might have been complications with his operation because now he has pain that he will have to deal with his entire life. I'm not sure if he actually understood the risk of this entire thing. And he has also been ostracized from his community. This system of organ trade, even though it is illegal, has become so bad that it's like a human pyramid scheme. They recruit one person to sell their kidney and every time that person gets someone else to sell a kidney, they take a cut. Pretty terrible stuff.