A quick search for images of "goth fashion" reveals a ton of black vinyl and latex. After doing a little research, I gained a more formal understanding of goth fashion and why vinyl and pvc is seen so often within the culture.
While the requirement of black color is used to show the "darker" side of sexuality and fashion, the preference for vinyl, pvc, and latex is an emphasis on the "otherness" of the culture.
Part of the goth culture is to be outside the mainstream. Since these fabrics are rarely seen in mainstream society, they fit the bill. The fabric itself makes a statement. It says - look at me, I'm different, it's ok to look different. Also, there is an emphasis on eroticism within the goth culture. Vinyl, leather, and latex are sexually charged fabrics, as are satin, silk, and lace.
Reading on wikipedia (everyones favorite place to research) revealed that Goth fashion is a calucated rejection to the unattainable "Barbie" or "Ken" look emphasized by mainstream media. Goth clothing often emphasizes or sexualizes parts of the body - such as corsets, tight sleeves or pants. Interesting stuff... check out this blog link about Japanese Gothic Lolita fashion to read more: