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Because it smoked as a child? Its mother didn't eat enough vitamins during pregnancy?

They really mean why was beer in short supply? The brewers, via the advertisement below, were keen to point out that it wasn't their fault.

The Government has found it necessary to limit the supplies of barley for brewing beer because of the acute world shortage of grain. This limitation, coupled with the increasing popularity of beer as a national beverage amongst all classes, is the reason for the continued shortage of beer throughout the country.

The reductions which have been made in the strength beer have partially alleviated the limitations in quantity which have been imposed. The output in standard barrels during the war years was restricted to that brewed in 1938/39, and was further restricted in May last to 90% of the output. The small increase subsequently granted has not materially altered the position.

Brewers have done their utmost to maintain the maximum permitted supplies of beer to the public; and they, and the retailers, have done their best to distribute those limited supplies fairly among all their customers. This they will continue to do. They ask the public to be tolerant of the difficulties under which the industry is at present working. The industry looks forward to the days when it can brew sufficient and better beer.
Issued by The Brewers' Society"
Western Times - Friday 01 November 1946, page 3.
There is a tacit admission that wartime beer was sub-standard: "when it can brew sufficient and better beer".

Reading the article, it sounds as if beer production in terms of standard barrels* had declined. But that wasn't true. Production, in terms of standard barrels, increased by 12% between 1939 and 1946. And in terms of bulk barrels, it was up 32%.

UK Beer production and average OG 1939 - 1947
Year bulk barrels standard barrels average OG
1939 24,674,992 18,364,156 1040.93
1940 25,366,782 18,738,619 1040.62
1941 26,203,803 18,351,113 1038.51
1942 29,860,796 19,294,605 1035.53
1943 29,296,672 18,293,919 1034.34
1944 30,478,289 19,193,773 1034.63
1945 31,332,852 19,678,449 1034.54
1946 32,650,200 20,612,225 1034.72
1947 29,261,398 17,343,690 1032.59
Brewers' Almanack 1955, p. 50.

It's impressive how well beer production held up during WW II. only in one year, 1943, was the output in standard barrels down on 1939. And then only by 70,000 barrels.

Though after 1947 it was a very different story, with beer production falling year on year for more than a decade.

* A "standard" barrel = 36 imperial gallons of a beer with an OG of 1055º.