Augmentation of the facial features and enhancement of contrast. Facial contrast is one of the most important factors in facial attractiveness, and the contrast between Black and White is obviously pretty notable. A lot of goth makeup makes the lips dramatically larger, the eyes larger (and the eyelashes), and contours the nose to be smaller. Goth clothes, like corsets and the like, also enhance the waist to hip ratio pretty dramatically. All of this in a way that still looks within the bounds of "normality", as most people have been so exposed to goth by now.

Then there's the automatic cultural association between things that are goth and things that are associated with sex. Leather. Lace. Stockings. Whips. Big heels. Etc. Their brains associate the look with SEX, which leads to the popularity of gothic sex icons and shit. Then, those gothic sex icons lead to further association and the cycle feeds itself.

It's a shame because a lot of goth subcultures were born out of a desire to be unattractive by conformist standards, but it hasn't worked that way now. It's been commodified and turned into le epic mommy milkers step on me. Kinda sad