For most people who have lived, this question is on-the-face nonsensical. Most cultures up until now have had some sort of value for purity or sanctity. The very idea of extracting an organ from one person and placing it in another prompts a feeling of disgust and revulsion. Many cultures impose a strong taboo on even touching a dead body. Western culture has largely rejected the ancient value of purity for a variety of practical reasons—not the least of which is to allow Western medicine to proceed.
I won't argue here that such a value ought to outweigh the practical considerations of allowing trade in human organs, but I will say that the value of sanctity neither is nor can be defended via reason. Valuing it is just part of being human.
There is also a very real social justice consideration: the people most likely to need to sell organs are also the most likely to need to remain healthy. The simplest way to get at the concern is to ask yourself how much it would take to sell, for instance, one of your kidneys. Would $10,000 be enough to give up a vital organ that you may want to hold onto for your later years? What about $1 million? If you are very poor, the first number would be tempting because it represents a large percentage of their yearly income. But I don't think the second number would interest me at all.
It's the poor of the world (and it's quite likely you, the reader, are quite rich relative to the entire world's population) who have the hardest time remaining healthy. And health is a much bigger factor in the earning power of the poor.
Again, this consideration might not be a problem in light of the benefits of a used organ market, but you can't just dismiss them either.