But the opposite isn't desirable either. Sure, you can make it more legal, but now these people are being systematically exploited for their organs and there would be very little reason for it to go away, meaning there would now be incentives to keep people brutally poor so that you can get their organs.
What if you made a social program that said "if you are desperate enough to sell your organs, come to us and we will give you resources so you don't need to (Conditions apply)." And in order to curb abuse, some small percentage of the applicants actually were required to donate an organ, so that the program wasn't exploited by people stealing resources from those very desperate people. If you take resources from this program, you risk actually selling an organ like you otherwise intended. Only truly desperate people would be brave enough to use the program. And if you think that compelled donation is also barbaric, even in a small percentage, it would actually be a lie, but one that some portion of the program's budget is dedicated to upholding.