Because SEX is what is popular. Woman want to look / feel sexy and lets face it "regular" panties make woman feel sloppy... Idk about briefs but from my standpoint they look feminine (which is fine, but I like my man wearing boxers)
Their not really - it is a preference both ways - a lot cannot wear either way - other than that everyone wants to look sexy --- even to give up comfort and health reasons to wear briefs and or boxers --- some guys have to wear underwear to accommodate their endowments
With women, it's because we want to feel pretty. Even if no one can see our underwear, WE know, so it makes us FEEL prettier. As for men's briefs, I've heard some men say they're restricting and uncomfortable Like their stuff is being strangled, lol (not my words). I've also heard they cause chafing during hot days.