Also, what caused the need in the first place? Smoking, drinking, obesity, diabetes, genetic defect? If the government mandates organ donation, then how much more control over our daily lives will they try to exert to make sure we keep our organs healthy?
If it is made the default choice, then how do the doctor know the organs are even viable and can be transplanted? Do the doctors at hospitals doing the harvesting have all the medical information on someone who has just been killed? Can they get that information in time? Do you really want to pass on the liver of someone who had/has hepatitis but who didn't bother to uncheck the option? How would the doctors know if they have/had hep without a full screening of medical records?
I don't know very much about how they can check such things. I personally am not an organ donor. I have way to many health issues and I am also not allowed to donate blood. But if I die in some way they could harvest my organs, how would those doctors know anything?
Making anything like organ donations mandatory or even making it the default option opens up to many bad things to happen that shouldn't.