Down Syndrome is a spectrum, some people can function in society, but quite a few cant. Why take that risk when you can terminate the pregnancy and start again.
Perhaps people have empathy and respect for human life rather than ending it just because their kid might have a disability??
I've had the opportunity to live and work with many people of varying abilities (and disabilities) and I'm so so glad every day that I see them that I had the chance to meet them and get to know them. That would never have happened if their parents decided they weren't worth the burden and murdered them in the womb.