Partly it's people reacting to the ugliness of the rich simply buying organs from the poor. But it's an aesthetic reaction to an otherwise mutually beneficial relationship.

One might wonder if these are exploitative transactions, and thus morally worthy of a ban, just as exploitative usury was banned in the Bible.

I would personally say that the transactions themselves shouldn't be banned, but that in an ideal society no one feel coerced into it, i.e. "I don't want to lose my kidney, but this is the only way to feed my family"