The Atlantic answered the age old question: Why do we wear pants?:
“Historically there is a very strong correlation between horse-riding and pants,” Turchin wrote in a blog post this week. “In Japan, for example, the traditional dress is kimono, but the warrior class (samurai) wore baggy pants (sometimes characterized as a divided skirt), hakama. Before the introduction of horses by Europeans (actually, re-introduction – horses were native to North America, but were hunted to extinction when humans first arrived there), civilized Amerindians wore kilts.”
So essentially it’s because it allowed for better movement, and keeps our balls (and appropriate lady parts for the ladies) from showing. This however doesn’t answer why we still wear pants. I don’t know about anyone else, but I can’t remember the last time I was on a horse (was likely a class trip when I was quite young).