Religious reasons, family not wanting that to happen, etc.
In short, only a few religions have anything to say against being a donor at death, and a few have something against being a recipient from a dead person.
If you think your religion might affect your decision to be a donor, check with a trusted authority figure and make sure.
A person's choice is probably always valid regardless of what it is, it's their body at the end of the day. In the normative sense I think everyone should donate their organs, but I'm not sure it can be reasonably codified.