There are three whole-chromosome trisomies of autosomes that can survive to birth: 13, 18, and 21. Do we know why these survive but trisomies of the other smaller chromosomes do not?
One simplistic model would be that there's a small fraction of genes that are particularly sensitive to 1.5x dosage, and these three chromosomes happen not to carry any of these. (In case you haven't guessed, I'm teaching human genetics this week.)
I think that @greally has the correct answer: that this simply follows gene numbers, which I had totally missed (also note that the severity of 13>18>21, consistent with this). Here are gene numbers by chromosome (from Wikipedia, based on Ensembl)
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