Conspiracy theories and actual corruption.

People are afraid doctors will kill them to get the organs. Doctors don’t want to deal with those accusations so will not take organs without clear consent from family. Some places are actually opt-out, doctors have enough legal protection and public trust is high enough that doctors will take organs without waiting for consent from family. In other places…

I remember some case from Poland where as a part of corruption case doctor was accused of killing a patient to harvest his organs. The patient was allegedly in a coma and the doctor disconnected him from life support to get the organs. This was clearly a witch hunt, all the other doctors got scared and transplantations in the entire country went down as a result.

So in many places people simply don’t trust the doctors and will not give consent. There’s also a lot of superstition and religious thinking involved.