This is what Michael Jackson has to say on the topic:
"When both Porters and Stouts diminished in popularity in Britain, why did they stand their ground in Ireland?
One reason may be that restrictions on the use of energy during World War I made it difficult for British maltsters to roast their grains.
These restrictions were not imposed in Ireland, where rebellion and independence were in the wind."
It sounds like a reasonable theory - brewers in England couldn't get their hands on the right malts and so were unable to brew Porter and Stout. Let's see how well it matches with the evidence from Whitbread's brewing logs.
In 1914 Whitbread brewed an impressive range of Porter and Stout: Porter, ES (Export Stout), LS (London Stout), SS (Double Stout) and SSS (Triple Stout).
Dark malts - brown malt and black malt - made up between 12% and 16% of the grist, depending on the beer type.
What would happen later in the war if, as the theory suggests, dark malts were in short supply? It would seem logical that either Whitbread would have stopped brewing Porter and Stout or that the amount of dark malts employed in their manufacture would have been greatly reduced.
Most breweries pared back their beer ranges in WW I because of government restrictions on beer strengths and shortages of raw materials. Barclay Perkins discontinued their bread and butter beer from before the war - X Mild - and replaced it with the low-gravity 4d government ale. Most stronger ales weren't brewed at all between 1917 and 1919. Whitbread was no exception.
Between April 1917 and April 1918 the Whitbread Porter brewery produced only two beers: Imp (Imperial Stout) and Porter. From April 1918 to April 1919, they only brewed Porter and LS (London Stout). The one beer that they brewed throughout the war was Porter.
Let's take a look at how Whitbread's Porter and Stout shaped up towards the end of the war. Remember that April 1917 is when the goverment began to really put pressure on the brewing industry with emergency legislation.
Well fancy that - the proportion of brown and black malt in the grist increased later in the war. In 1917 it was around 18%, in 1918 and 1919 between 18.5% and 22.7%. You know what, it doesn't look as if they were running out of brown and black malt, does it?
The terminal decline of Porter was well underway before WW I broke out. As for other styles, wartime restrictions forced down the gravity, but they don't appear to have had any more effect than that. Porter's popularity continued to ebb away during the 1920's and 1930's and by the outbreak of WW II it had all but disappeared.
So don't blame Kaiser Wilhelm or Lloyd George for Porter's demise. The answer is far simpler - public taste had moved on.