They have a significant reduced live span and are far away from the normal population. They also can't reproduce (at least woman, only with very high risk).

Most people don't want to be unique, they want to be a part of the rest of the herde.

Its objectivly better to not have down syndrom.

And before i get downvoted: My stand doesn't mean i look down on peole with down syndrom. These two viewpoints are not exclusive. The same for the decision to abort a fetus with down syndrom doesn't mean that someone decided this, would look down on people with down syndrom.