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Capital punishment exists to punish crime, not to aid scientific study. Subverting the purpose of execution can create a perverse incentive structure, wherein a need for more scientific study may drive judges to sentence more convicts to death row than they otherwise would.
If they die, we were going to end their lives anyways so why not put those extra humans to good use?
Death row inmates are not representative of the general population, which may largely eliminate their utility as subjects in scientific experiments.
Part of the constraints of capital punishments in modern society is that they are quick and humane. We don't torture people.
Many forms of experimentation on criminals would likely be equivalent to torture and are inherently unethical because of the recurring harm they may cause.