[User3456335]

My point is that perhaps there is a reason that this chromosome causes this issue when an additional copy is present. If other chromosomes do not cause the same issues, why not? Why would that same reason not hold for chromosome 21?

[Retric]

You have it backwards here, Down syndrome is unusual for being viable not because the symptoms are so terrible. Most chromosomal anomalies result in a non viable egg / sperm which doesn’t result in a live birth. The majority don’t even last long enough for someone to notice they were potentially pregnant.

Turner syndrome is another example without the mental issues: https://en.wikipedia.org/wiki/Turner_syndrome

In the larger context, there may be some long term advantages to chromosomal anomalies because their number varies so much between closely related species.