Because by the third trimester, an unborn child is capable of surviving outside the womb. This means aborting the child is intentionally killing the child when a less destructive option is on the table to end the pregnancy, i.e. simply delivering the baby.
Most of these fetuses have birth defects. Do you think we should "simply deliver" babies with deformities or down syndrome?
Yes, doing otherwise would say that people with Down Syndrome aren't valuable and shouldn't be allowed to live. Practicing eugenics on people with disabilities through abortion is despicable- even beyond the abortion issue itself.