Here’s a leap that's hard to fathom: Bimbo in Italian means "baby, a male baby" (female is bimba). And, somehow (around 1920), the word in English came to mean "a brutish or doltish man." However, it quickly came to also mean "a woman of questionable virtue." Then, sometime around the 1950s, it became used almost exclusively to mean "a dumb, but likely attractive and virtuously questionable woman."
Why did bimbo, the masculine form, become a derogatory word for a female? And, why did it cease being a derogatory term for men?
(The ‘90s sitcom Seinfeld featured an episode that coined the word mimbo, for a male bimbo. Maybe we just need to add that word to our lexicon to balance this one out.)