Earlier members of the military didn't wear trousers, seeing them as the clothing for barbarians, or effeminate like their Greek predecessor.
Pants are women's clothing!
That my be assuming too much, but the Greeks seemed to think so, and it carried over to Romans.
The Greek's ancient histories tell stories (often called "myths) of a race of women who managed to form a society of women in which men were not allowed any power. They were quite skilled at war and horse riding.
The stories were quite numerous, and have been discovered in quite a few different places and by multiple authors. The Greeks both feared and despised this race of women, yet declared them to be beautiful and strong. You may have heard of the "Amazon" women.
The stories show that women fought for liberation from being subjected by men way back in ancient times. They were written by men who apparently sought to show the triumph of men. For though they talk of the women as mighty warriors, they never write of any battles that the women actually won, but rather of men overcoming them.
Did such a race exist? There are various opinions.
Some say they were actually beardless Mongolian men, mistaken to be women. Some say they were just myths to show that men will always end up as rulers as the Amazon women were eventually defeated and wiped out. The women received "bad press".
But there is evidence that suggests they actually existed.
1. Ancient historians wrote about them, like Homer, Heredotus, Plutarch and Eusebius
2. in the Altai Mountains of Siberia, an excavations began in the mid 1990s. The archaeologists unearthed the burial grounds of an ancient race of people known as Pazyryk. The Kurgans, as the log-barrows they were interred in were known, have revealed several mummified remains of mostly women. One such Kurgan contained the well preserved remains of a woman who had been interred with her resplendent feathered headgear, intricate body art and sacrosanct artifacts. She has since been called the "Siberian Ice Maiden".
3, Several other kurgans were excavated and revealed mummified women displaying scars and wounds perhaps inflicted in battles. Most remains are bow-legged, an indication of years of riding on horseback. Bizarre but true are the remains of horses found at these Kurgans, some sacrificed and propped up on stakes, others interred with the dead.
Whatever the truth is about them --
The Greeks seemed to believe they existed, and yes, the Greeks credited these women with inventing trousers.