Peter Turchin points his finger at horses. Daniel Lametti summarizes:
Take, for example, the Romans, who traditionally wore tunics. As Roman soldiers began to use cavalry in battle they started covering their legs to make riding more comfortable. Knights of the middle ages, who were often on horseback, continued this practice for the same reason. Because of this, "wearing pants," Turchin writes, "became associated with high-status men, and gradually spread to other males."
The Dish 