For instance, in black-and-white photos of cowboys and old politicians, you can see a number different hairstyles, most of which could be characterized as long-ish, and a great variety of beards, mustaches, sideburns, goatees, etc.

However at some point before the cultural revolution of the 60s and 70s, short hair and clean-shavenness became such a norm for men that wearing long hair was in and of itself a protest against the Man, the system, etc. Men with long hair were seen as transgressing their gender roles, even effeminate or gay, which was persecuted back then.

As an example, I recall reading in one John Lennon biography of an account where the newly famous Beatles met their long-time idol, Elvis Presly. However, the meeting turned out to be awkward and uncomfortable, and Elvis later characterized the Beatles as something to the effect of "a bunch of guys with f***y haircuts", impuging their masculinity and sexual orientation. This was the mop-top Beatles, who often performed in suit and tie outfits, not the long-haired hippie Beatles of their later period. So even a haircut that would not be considered "long" on a man by today's standard was norm-violating enough for Elvis to deride in slurring language.

When, how, and why did short hair become so important to American males before the cultural/sexual revolution of the 60s and 70s?