I beleive the growing and oasting of hops was/is fairly manually intensive, so insufficient labor force? As you infer, the Germans probably scooped up much of the available hop and barley crop too.
Sugar's probably not easy to come by in wartime Europe either.
The strength of beer in the UK fell by 20% during the war because of shortages and a third of the 1940 hop harvest burned after a single air raid on London as well as having problems getting people to go out to Kent and harvest what was left while the Battle of Britain was fought above them.
Brewing in WWII
War impacted British brewing both directly and indirectly. Government action is a good example of a direct influence.
www.beeradvocate.com