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 embro 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.
Why do you think stem cells or pre-implantation embryos are the victims?
Didn't I just make that clear?
Scientifically human life begins at conception. The only way to justify ending it with the goal of promoting scientific advancement is to dehumanize the early stages of that life and then claim it has no value.
Such dehumanization is a category error witnessed in its most extreme form in the Nazi experimentation but also prevalent in modern research on human embryos.