Because, regardless of the mom's circumstances, the baby's "innocence" has never changed. It still doesn't "deserve" to die.
Yet in one situation, people would bend the rules for the mom, in the other they would not. Logically, then, it's the mom and her situation which is the basis for the decision.. and NOT the often-focused-on ideal of the "baby being a human life."
The baby's humanity has no bearing apparently on society's decision, it's all the mother's situation, yet the position is called "pro-life [in terms of baby]." It's not, technically, if one makes exceptions for the mom's circumstances.
(I am not criticizing a particular stance, just pointing out a mislabeling/inconsistency as it is practiced.)