They cover your legs in the cold, so pants first appeared and persisted for WOMEN because they were practical. But they eventually came to represent power, equality, and freedom. That's the history in a nutshell.
Add the "tomboy" phase in the early chapter of a woman's life. She didn't need to understand the history at all, all she needed to know was that she couldn't play on the monkey bars while wearing a dress, so she begged her mom for permission to wear her jeans. A pair of jeans: clothing and playground leveler.
Now ENTER the striking images of LAUREN BACALL and KATHERINE HEPBURN wearing trousers and owning it, and the MODERN WOMAN is shaped.