2. The high status of female Love Martyrs among females (and, sometimes, among males).Let's be honest, girls love to drown themselves in a sea of hormonal emotions. Most Fan Fictions featuring a Draco in Leather Pants often make him a Bastard Boyfriend who turns into a Reformed Rake in the end, through The Power of Love. Many teenage girls want be on an emotional rollercoaster and feel constant insecurity about whether or not their boyfriend loves them. It makes them feel alive and gives them the feeling that they have a purpose in life. Being strong enough to love a man forever, no matter what he puts you through or what he does to other people makes you a respectable and admirable woman, according to many teenage girls. Hence the glorification of Love Martyrdom, which is what most villains guarantee.
3. Reverberation of high status.Villains are often ambitious. Since most of them often become kings, emperors, or have some form of authority, girls dream about being the respected dark queen at their side, whose high status ensures that no one will ever be disrespectful to her or disobey any order she may give as a the Dark Lord's lady. In other words, the freedom of being a bitch and not getting any flack for it. House proves that many men like being jerks and get away with it. Women are no different.
4. Evil Is Sexy.It's true for men. Femme Fatales and Vamps are often considered "hot" by Fan Boys. While the heroine may look pretty, she doesn't ooze sexuality like villainesses tend to do, since, as we all know, a sexually active woman is not wife-material. Since this is inverted for males, the sexiness male villains ooze is often inherent and switches on many girls' desire to be dominated and literally owned by a bad man. Love Martyrdom often involves Sex Martyrdom and other kinds of unsavoury things. All of this is nullified if the villain is physically unattractive, though.
5. I Can Change Him.The inverse of #2, in this case it's the feeling that the girl's (or guys as well) love is capable of changing the Draco.
6. Double Standard and female villainsAs one can notice, we mostly talk about girls and women when it comes to people who invoke the trope. The reason? Count the female villains who get sexialised for the same deeds compared to the male ones. It seems Fanboys are hardly as sensitive to the Draco in Leather Pants effect as Fangirls are. If a female villain is sexy enough, she may be turned into a Lust Object but rarely will boys and men wax eloquent about marrying or changing her. And, as one coulkd expect, girls and women will just hate her for being an evil bitch when most of them would be fapping at the sight of a sexy man doing the exact same thing.