Breastfeeding vs Formula feeding

Tanya
I currently formula feed my 2 month old because my milk supply never came in. My pediatrician said we need to feed him every 3 hours during the day and we can go 4 hours at night.  My preemie came 2 months early and was due on 1-1-2017. How come some babies are allowed to go on demand and are allowed to sleep through the night. I forgot to ask why we can't go on demand. Is there a different schedule between breast fed babies vs formula fed babies.