It ultimately is religious belief.
Religious people believe the soul enters the body at conception, granting personhood, so abortion is murder. They also believe that people put to death will go before God, where they will be judged as evil and sent to Hell for eternal punishment.
Everything else is just window dressing.
Is it universal that pro-lifers only make exception if the life of the person carrying the pregnancy is in danger? I’ve seen pro-lifers who make exception for rape and incest, and others who would advocate banning it in all instances, even when the life of both is at risk.