There's no objective reason not to allow it. Think the reason it isn't here in the US (but is in other countries) is it's gross.

Plus, because of how fixated on costs we are, if we allowed people to sell off akidney, there's the concernt hat then society may have to pay more for those people if their sole kidney stops working right. So many of our laws governing safety like seatbelts, and motorcycle helmets are because of public costs, not any moral concern about people's welfare. Would assume a similar issue being at play with selling organs. We can sell blood and semen afaik because they're self-replenishing. But organs are finite.
Plus, selling off organs creates a real dystopian concern if the underclasses became used as organ donors. Movie "The Island" shows this well.