I don't understand why more people who need kidneys don't just go to Iran and buy one.

I suspect that most people aren't aware of the option; I certainly wasn't. And a U.S. citizen might still be breaking U.S. law by doing so even if it's legal in the other country. (Several U.S. laws work that way; I can't speak for the laws of other countries.) I also don't know how hard it is to get a visa to travel to Iran.