You have to remember that you are not the only patient in the hospital. A Caesarean section requires a lot of setup including OR, anesthesiologist, ensuring adequate care for the newborn, etc. Any of these can cause a delay. Of course there is other cases triaged by urgency like crash sections. It would make no sense to show up to the OR 30 minutes before you can begin operating. An elective Caesarean section will always get bumped for emergencies.