Probably because the poor would have a lot of trouble getting organ transplants. Under the current system, you're put on a waiting list until your number is called. If others can bid more money on organs, you're out of luck. I can't imagine insurance companies being overly keen on bidding for organs.

I really don't want to think about issues arising from people selling off their organs to make ends meet (or to settle gambling debts). And let's not forget human trafficking. Instead of just forcing people into prostitution, you could also harvest their organs.

I think there are several thorny problems and issues of consent that complicate this matter. Losing an organ is permanent.