In part this was because, for some silly reason, leaders wanted to keep their soldiers and sailors relatively sober. In 1915 David Lloyd George famously declared that drink, not the Central Powers, was Britain's greatest enemy. "We are fighting Germans, Austrians, and drink, and as far as I can see the greatest of these deadly foes is drink."
The men at the front may have disagreed about that, but they had little say in the matter. They did mange to get weak French lagers and a rum ration. After the war, the medical officer of Scotland's 4th Black Watch Regiment told a hearing on shell shock, "Had it not been for the rum ration, I do not think we should have won the war."
For Lloyd George, then Minister of Munitions, drink was if anything a greater foe at home than at the front. He and many others in authority feared that drunkenness would reduce workers' productivity, depriving the fighting men of needed war materials and supplies.
The authorities were especially concerned about heavy alcohol consumption among munitions workers, for good reason. Mistakes and misbehavior in munitions plants could lead to deadly explosions and delays in production. Hundreds of munitions workers died in explosions, most of them women. (below: Charles Ginner, "The Filling Factory, 1918)
Starting in 1914, various Defence of the Realm Acts (DORA) gave the government sweeping powers to control many aspects of British life, including the production, sale, and consumption of alcoholic drinks. One government order prohibited "treating" or buying a round for other patrons. All drinks had to be paid for by the person consuming them.
Some changes had long lasting effects. One severely restricted licensing hours, the time when pubs could legally serve alcohol. Opening hours were limited to the early afternoon and early evening, from 12:00 to 2:30 and 6:30 to 9:30. Violation could lead to loss of the pub's license. These restrictions remained in effect until 1988.
Government propaganda also encouraged individuals to restrict their own drinking times, as in the poster below:
The wartime government also significantly increased the taxes on alcohol. The price of a pint roughly doubled even as its strength fell. In 1918, a bottle of whisky cost five times its price in 1914. The consumption of alcohol fell by about half during these years. Arrests for public drunkenness fell even more drastically.
How much all these changes affected the war's outcome may be debatable, but they certainly saved some lives, and changed British social life in long-lasting ways.