Why is aneuploidy usually lethal?
What causes zygotic termination in aneuploidy often isn't an issue with proteins or other gene products i.e. being "too sick" to live. It's that the zygote is basically "self error checking" and if it encounters a serious genetic error (e.g. aneuploidy) it self-terminates, as it would likely not survive anyway. Sometimes this self-termination fails, and the zygote develops. Depending on the specific aneuploidy, it may not make it to term, or perhaps it will, and you will have a child born, albeit the child will usually have a decreased quality of life/life-span.

A similar mechanism works in your somatic (non-reproductive) cells everyday when they replicate and divide - if they detect and error in their DNA that cannot be repaired, the cells SHOULD kill themselves. Sometimes this fails, and can give rise to cancer.