Because both involve something monstrous that is justified by dehumanizing and reducing the victim to the level of mere object.
With stem cell research the experimentation and dissection is justified by claiming the embryo is not a real human life. With Nazi experimentation the medical experimentation was justified because the victims were not true valuable humans just Untermensch sub-man or subhumans. Their loss benefited the Übermensch with scientific knowledge and thus their sacrifice was justified for the greater good.

Its the same logical error in both cases.

As I said earlier there is something particularly monstrous about creating human life. Deciding its no longer wanted or needed for some convenience or economic reason and then instead of nurturing that life into birth and adulthood choosing to kill and experiment on it for knowledge and profit.