1. You don't have bodily autonomy now.
2. They can fight your decision.
"Over in the United States, the criteria for donating a body to science is basically the same and require you to sign a few forms signalling consent prior to death and, again, it’s a good idea to inform your family of your wishes in regards to body disposal because, as in the UK, the ultimate decision of what happens to your body rests with your kin. As the Forensic Anthropology Center of the University of Tennessee puts it:
Regardless of what you have arranged, signed or instructed, your family or next-of-kin has the final say. We will not fight your family for your body. We urge you to convince your family that the donation is what you want at your death."
http://www.todayifoundout.com/index....20your%20death.