Not that this qualifies as a "good reason" so much as my hamster-wheel of devil's advocacy trying to turn (I actually agree that people should be able to sell their organs (and end their lives with dignity (and that both of these things would make healthcare far more efficient and cheap (wow I'm using a lot of parentheses))), it makes me ask a few questions...
1) how does the traditional health insurance reimbursement market and/or medical field treat organ prioritization? Do they have to pay for these?
2) Is this prioritization system mandated by law, or just the norms of the medical community and insurance companies?
3) If a more capitalistic system developed, how would insurance companies and health providers work with it? Prices would go up (obviously... from nothing) on organs. They would now have to pay for them... dearly. Would they change their prioritization system? Would they set a max on expenditures for organs?
(prepare to have your minds blown)
Imagine how much more effective the donor market would be if it operated on more capitalistic principals (as "icky" as it may seem). How many more people would mark "organ donor" on their licence if they knew it meant an extra $30,000 for their family if something happened to them? How many people would be willing to give up a kidney for $40,000, which would solve two problems at once?
It is a friggin' tragedy that more market-based principals aren't used for organ donations... How many people have died as a result?
(PS... I'm still an advocate of a social safety net you heartless capitalist pigs, you!)

"Men did not make the earth. It is the value of the improvements only, and not the earth itself, that is individual property. Every proprietor owes to the community a ground rent for the land which he holds."
- Thomas Paine