homonculus is right,
transplants are not time critical in the same way like HEMS. They take the organs planned and in coordination with the receiver hospitals, but mostly at night. Usually they have more free operation rooms and medics at night and the daylight business can go in the normal way. Thats the simple reason for night transplant flights.
The problem is, they don't ask the flight crews about the weather. We are just service providers. Simple call and we have to come.