It'a pretty simple logic. The woman dying is an act of god but the abortion is a deliberate choice to take a life ie a sin. I don't agree with them but that is the thinking. (ETA - referring here to the religious nuts who won't abort a pregnancy to save the mother, not the religious nuts who want to execute woemn who have abortions)
Beyond that, the whole thing is based on the belief that if something bad has to happen, better to save a soul without sin than one with. Since the foetus has not had the change to sin yet it always wins against a human with a lived life. They prefer the potential of perfection implied by a tabula rasa than the messy compromises of existing in the world.
This is also why they don't care what happens to the "saved" kids after birth. Birth takes them from perfect clean slate to messy needy human, and the longer they live the more tainted and less worthy of consideration they become.