Doing so, leftists will then be admitting that what they are killing *does* feel pain. They don't want people to focus on what is really going on with the business of "big abortion".
I think a lot of people in favor of allowing abortion already concede the possibility that fetuses feel pain. Admitting the possibility of pain is a far cry from admitting humanity in a fetus. I have a slime mold that responds to negative feedback/pain and it can almost be outwitted by a dead cabbage.
Fetal pain laws will, in many states, simply lead to pain drugs or “humane” procedures that will let many people think they’ve “done something” and that any problem is fixed.