So why would you you want to use all your opening hours? It did make sense
"Closing during Permitted Hours
A brief review of some of the more striking points in the reports of Brewster Sessions is given on page 75 of this issue. One of the lessons to be drawn from these reports is the unwisdom of licensees who, having too little beer to meet the needs of their customers, have mil. kept their houses open during the whole of the permitted hours. In some districts there has been an understanding with the licensing justices beforehand, in others the justices have not hesitated to criticise strongly such closing. It is not only the closing of houses in the evening at an earlier hour than that provided under the Licensing Acts, but at such times for instance as the afternoon hours allowed on market days. The Brewers’ Society have on several occasions deprecated such a practice, because in their view it is not the proper way in which to deal with the undoubted difficulty in which the licensee is placed where supplies are short—as in most districts they must be under war conditions. Wherever possible retailers should ration out their beer supplies over the whole week rather than meet the full demand at certain times and close at others, and they should spread their daily sales over the whole of the permitted hours, particularly the latter part of them, and at week-ends. There is something to be said for special arrangements in districts where circumstances such as the withdrawal of transport at an early hour would prevent the staff reaching their homes, but even here any early closing should be the subject of a definite arrangement and should not be at the whim of the individual licensee. The first consideration should be given to the fair distribution of available supplies to the public, and a policy on the part of a licensee of first-come-first-serve is, to say the least, unfortunate, and one which may have repercussions later on."
The Brewing Trade Review, March 1943, page 65.
Simple, really. If you only have enough beer to last for half your permitted hours, why bother opening for all of them?
How exactly was a publican expected to spread sales cross the day? Limit how many pints each drinker could have? Something which would be sure to piss people off. Far easier and less stressful just to open for fewer hours.