The utilization rate is so high that specific planes and crew are needed in specific times and places. If plane going from A -> B is late it cannot become flig

The utilization rate is so high that specific planes and crew are needed in specific times and places. If plane going from A -> B is late it cannot become flight B -> C. If crew for this plane are stuck in the ground long enough, they have to wait til the next day to fly as they‘re only allowed to be awake and work for so long.