Because it's attitude towards life, new life, is so relativistic and unloving.
Sophia wrote, in the context of a blogpost on prenatal testing:
Forcing women to give birth to an unwanted fetus is morally reprehensible
So for poor-choicers: Forcing a woman to give birth, i.e. to not kill, their child is morally reprehensible.
But not accepting your child unconditionally is not morally reprehensible?
The prochoice movement does not expect women to love their children unconditionally.
That's what it boils down to.
Does no one see the problem here?
Some of them might try to construe killing their offspring as a merciful act.
As if killing your child because of Down Syndrome is really an act of mercy.
You don't destroy people you love. You try to help them get through their difficulties and improve themselves.
We've totally lost the meaning of love.
But poor-choicers don't get it. They do not understand the true meaning of love. They think anything they construe as "love" is loving because they erase some negative effects. It doesn't work that way. Love has its own meaning and its own rules.