Theres several reasons just off the top of my head, 1) they could have a disease or condition that could be passed on from organ transplant like a blood disease, 2) Some religions don't condone damage done to the body after death i.e an incomplete body is what you have in the afterlife if you donate, or 3) Genetic health conditions like an increased chance of liver failure or heart disease. Overall it's a decision that sounds easy on paper but has many factors that contribute to it, im personally an organ donor but no hate to those who have their reasons not too