In Candide, why did the female characters get the worst treatment?
It's difficult to answer this question with any authority when addressing such a satirical work. Voltaire is either commenting on what he sees in the plight of women in 18th-century Europe, or his depictions of women fit the literary typifications of much western literature: woman as property; desirable virginal beauties; used-up vessels subject to the whims and desires of men.
Indeed, women, too, are subject to the desires and capricies of those in power, Cunégonde and her maid being perhaps the best examples. Each are admired for what they offer in a physical sense. Candide is always thinking about the physical beauty of Cunégonde in an ideal sense, while the maid's body is used in a very literal sense (remember the buttock)
Women's bodies seem to belong to men, both ideologically and materially. This seems to be fact in Candide, like many of the other heinous ideas presented in the text. It reminds me of Ovid's observations in many of the stories from the Metamorphoses: that beautiful woman always seem to become victim's of men's desires.