Trolley problem indicates most people (myself included) would kill one person to save 5. This means it's acceptable to cause harm if you prevent more of it. This position is defined as pro intervention.
Homeless problem (is it better to refuse to give a homeless person food and let them die painfully or shoot them in the back of the head, ending their pain?) indicates that most people prefer painfully letting someone die vs killing them painlessly. This means it's acceptable to cause harm if you prevent more of it. This position is defined as anti intervention.
What is the difference between these 2 situations? The trolley problem requires less sacrifice than the homeless problem.
Low sacrifice = Pro intervention. High sacrifice = Anti intervention.
The abortion debate is certainly very high sacrifice, so it takes the anti-intervention position (it's better to painfully let someone die than to painlessly kill someone). So when people say, "How come pro lifers only want to prevent fetal death and not the death of foster kids?", well now you know why. They believe letting someone die (poorly funded foster care) painfully is not as bad as painlessly killing someone (abortion).
