Steve M

Great least of reasons in posts 53 and 54. I'll add another: related to maximum crew duty time, there's also minimum crew rest time. If it's the last flight of the day, the crew may be overnighting at the destination and then operating the first flight of next day on the same aircraft. Depending on how long they have between arriving the previous night and departing the next morning, the crew may be up against their minimum rest time, which means that delays in arriving the previous night (or perhaps any delay at all) cause the first flight the next day to be delayed. Any passengers on that flight that have tight connections must be weighted against those that would be missed if the previous day's last flight is not held.