As an unabashed fan of bodice buster novels I'll tell you why ladies didn't wear underpants. It was considered scandalous! Even ladies of the evening would have blushed at the thoughts of wearing panties. It was beyond the pale immodest. What ladies wore were crotchless bloomers. (And here you thought Fredericks of Hollywood invented that little number. No no no.)
Makes sense when you think of it. A lady could do her business with the crotchless bloomers. And no one could get a peek at anything. Not that they'd want to, but it wouldn't be possible, would it? Whereas with panties as we know them today....got to pull them down...and what if pulling them down led to keeping them down...which could lead to...
As long as we're talking women's history as it relates to fashion, I'll also tell you why hemlines rose in the 1920's and why it was in that decade that sweet young innocent things turned into wild flappers, drinkin' and smokin' and cussin'. It wasn't because fashionistas got tired of hemlines that swept the sidewalks. It was because of the invention of Tampax. Without the monthly bulk between the legs, voluminous skirts and bustles became a moot point. Tampax screamed for short skirts and swinging hips. BTW, I can't prove this with any published research because it was told to me by the elderly mother of a friend, who knows it because she lived it. And yes, she was one of those flamboyant flappers in her day.