The reasoning behind this is that on many flights, especially when full, the hat bins max out on volume. It is not unknown to have 10-20 bags have to go in the hold at point of boarding, with commensurate delay while they all get tagged and loaded late on. Admittedly, better regulation of bag size at the gate would help, but thanks to online check in, often it seems the first person to actually evaluate someone's carry on baggage size is the Cabin Crew member at doors 1.