(via https://en.wikipedia.org/wiki/Blackletter)

Moustaches were mandatory in the British army (and shaving them off a punishable offence) for about sixty years, up until the middle of the First World War, where the rule fell out of favour in no small part because too many of the new recruits were too young for anything more than peach fuzz.

The Victorian trend for really big bushy beards emerged in part out of mass public admiration for British soldiers returning from the Crimean War, who had grown long beards to help cope with the extreme cold over a very long campaign, after Queen Victoria noted them with some admiration. Though it probably did land at a time where shaving was somewhat out of fashion.

"So it was because German elite airmen needed oxygen at the altitude at which they flew, within the airplane. I suppose because the cabin was not pressurized. They had to be able to get a good tight fit on the masks, so the only facial hair they could have was a toothbrush mustache, because that didn't make the mask leak.

So in solidarity with elite airmen, to show they were just as masculine despite having so little facial hair, Hitler wore that mustache always."