The fact is that a huge number , probably most, of the people signed up to donate will die in circumstances that make donation of major internal organs practically impossible. Unless you die ( and not of active cancer, hepatitis, HIV or any other transmittable disease) in a hospital bed on life support then it is unlikely your organs will be useable.
Which rules out anyone dying at home, in a care home, in the street or at work of an accident, heart attack , massive embolism or stroke. Rules out anyone dying of massive blood loss or trauma at the scene of a RTA , or as a result of violence.
So the pool of potential donors has to be huge, in the hope that at least some of us die in a way that makes us viable donors. I hope I can donate, if not major organs then skin, corneas, heart valves or any other bits that someone else can use, but I am realistic about it. It probably won't happen.