Don’t take my word for it but I my opinion is that the turning point was the first world war army reforms. Up until then well groomed beards were a sign of being well of and a point of pride, and you can tuck your hair in the bonnet so having long hair is not an issue, can even be advantageous!
The first world war changed many of those -uniform colors, changing the bonnet for a helmet, for example - and implanted a “need to standardize” that among other things, standardized the look because they were using these kind of details to identify officers! And as things happen, these men come back from serving, and import the “clean shaven, buzz cut” as the “thing that makes men, men”. By the second world war there were no amazing beards nor moustaches in the ranks.
just a sample:
First world war
Second world war
So yeah: clean shaved and buzz cut is really more of a mid 20th century thing 
