It's quite simple there is not enough organs to go around. It would be great if there were and maybe one day soon we will get to the point where we can grow organs specifically for people relatively cheaply. However, not supply does not meet demand and so the people in charge of choosing who gets an organ have to make very difficult decisions. Decisions they don't want to have to make, but do. They have decide who will make the best use of this organ. Now yes you can't completely investigate a person's life, but they do have information about patients and they have to make the best decision they can.
If you have patients who are vaccinated waiting for an organ and a patient who isn't, then in terms of who is looking after their health the most, the evidence only points one way.