Most Helpful Girl
The convention goes back to ancient times, when Roman men were judged harshly for having hair that required extra attention. I guess men also worked in the outdoors and on farms and in fields etc so it was always convenient for them to have short hair.
Most Helpful Guy
It hasn't always been like that. In certain time periods and areas in the past having long hair was in fashion for both men and women. Both in the Viking period, 1600s Salem - Massachusetts, Ancient Syria (the Assyrian time period) and ancient China for example having long hair was both in fashion and the norm for both genders. In some places having long hair was a status symbol and if you was poor or a slave you had short hair. So having short hair and being clean shaven hasn't always been seen as wealth, well-groomed, cool etc.
Because men did physical labor and went to war it was more practical with short hair and being clean shaven. Therefor having short hair became modern for men. Both the ancient Rome and during the WW1 people saw the benefits with short hair. Easier to wash, dry faster, easier to maintain and don't stand in the way. Women usually didn't have these jobs and therefor didn't need to have short hair for practical reasons. Long hair later symbolized both femininity since it shapes the face nicely making it lesser square looking, health (Men are known for being visual) and beauty. It made it easier to see the differences between the genders and heterosexual people are attracted to the opposite gender, so gender differences are attractive.