Because Little Johnny's mother is going ballistic with the lack of news. Little Johnny on the other hand think it's a hoot, so long as their mobile phone charge holds good. But the other factor is that yes, for an individual passenger they can be usually slotted on to the next day's flight, it's a harder proposition if there are 32 school children involved. Moreover it's no big deal if you have to connect in MUC or FRA or CDG. Technically BA should boot off almost anyone else on the next service given that the passengers are children, but that then pushes multiple problems down the path. Hence a coach is magicked up.