That is different because down syndrome is not hereditary. It's caused when chromosomes don't properly separate due their tangling caused by aging of egg cells.
Plus, if it were a hereditary genetic condition, abortion would eliminate those genes from the population reducing the occurrence over time.
Would you agree that there are probably some genes, especially on the mothers side which would increase the likelihood of the foetus having down-syndrome?
Or some genes on the mothers side which will increase likelihood of engaging in behaviour that increase the chances of the foetus having down-syndrome? (Smoking etc...)
Because if so, then a huge evolutionary pressure has been lifted from having a down-syndrome child. In the past having such a child would incur immense cost. There are of course difficulties birthing and sustaining an evolutionary dead-end which would reduce the birth-rate of down-syndrome having couples right?
Therefore, genes which increase the likelihood of having down-syndrome will proliferate much more in world which aborts down-syndrome foetuses right?
Plus, if it were a hereditary genetic condition, abortion would eliminate those genes from the population reducing the occurrence over time.