I think there are many reasons for this. One of them is that opposition to abortion is painted as automatically a "religious position" i.e. not based in reality or philosophy.
I had related discussion with a left-learning pro-life blogger. He wondered why Canada's pro-life groups weren't more welcoming of "alternative" pro-lifers.
There is a pratical and a philosophical reason for that.
Although I believe that my pro-life beliefs are demonstrable by reason alone, I am driven by faith. I believe that a society suffused with more faith will be a more pro-life society.
Atheists generally oppose faith. They don't want more religion in their lives.
It's a little bit contradictory to work for a group that one fundamentally disagrees with. So it wouldn't make a lot of sense (from a purely logical standpoint) for an atheist to join a pro-life/pro-family/pro-God group.
On a practical level, religious right-wing pro-lifers cannot be secular and/or "progressive" in their stead.
I can't start a secular pro-life group. Not with my religious motives and drive. When you live your life in a faith-filled way, it pervades what you do. I think a lot of pro-lifers feel the same way.
If "alternative" pro-lifers want their own group, they will have to start one. I welcome their participation in fighting for fetal rights. It would be a political boost, because it would make people comfortable with the idea that you can oppose abortion without being fundamentalist Christian.
I am confident that there is more opposition to abortion in Canada than meets the eye. I am also confident that if Canadians were to be educated about the real nature of abortion there would be even more opposition.
It's a really good column. Read the comments, too, they're very interesting.