Because a common occurrence when your poor is: You need $500 to pay for some debt now or a chain reaction will destroy your life. You've taken as much as your family has been willing to lend you and you've taken as much as the system (as much as you can understand it) is willing to loan you. You don't have that money.

Luckily, someone is offering $1,000 for a kidney and you're compatible! You can fix your car / post your bail / pay the loan shark, with $500 left over!

You're pretty much forced to sell your organ at this point. And this point is reached all the time by the economically disadvantaged.

Of course, without the last resort of selling body parts they'd be even more screwed, so I don't know the right thing to implement in this case. Just wanted to answer your question.