Speculation: skin grafts can be grown basically endlessly from a donated foreskin. Circumcision is more common in the US than in Brazil. As for other organ shortages, well, if a single foreskin can produce even ~100 square meters of skin grafts, you don't need very many of them to meet a country's skin needs. And ~100 m^2 of grafts per foreskin appears to be quite the lowball: http://archive.wired.com/science/discoveries/news/1999/02/17...

I'd guess also cosmetic surgery in the US produces quite a bit of human skin, although that's not expandable into skin grafts like the foreskin technique you link to.

You can donate your skin that is being removed after heavy weight loss: http://www.livestrong.com/article/126643-donate-skin-after-b...

I can imaging that this is a big source of donated skin.

This is a complete guess. I'd imagine, though, that it's significantly harder to abuse or destroy than other organs. Internal organs can be bad candidates for transplant due to a variety of dietary, lifestyle, or health factors. Skin seems subject to fewer of those.