because as player, we don't have the allowance to use rear units (reserve or 2nd echelon) to move into the vacated hex neither the attacking untis can move into such an hex and 1 hour is too much for that.
In your first campagn as german, crushing britsh is easy but if they rout at the start of the 1 hour time, you lost ALL that turn doing nothing beause you can't move.
you can't enter vacated hex neither exploiting cause your further move is halted/cancelled regarding the stacking rule. rear unit can't move into first line troops and first line troops can't move cause no orders to move.
now 1/2 a turn allow better ue of exploiting manoeuver which is the main purpose of the desert war