Young kids who feel like they don't fit in are big consumers of music, and BVB and BOTDF are favorites of that demographic right now. A band that wears weird clothes look like outsiders and that gives them an instant "connection" to kids who feel alienated and different than their peers. That's pretty much all there is to it, I don't think it has to do with the music at all. (except that the graphic sexual lyrics have the added bonus of making kids feel rebellious)
yea i know that's why it's appealing to them, but at the same time there are numerous better bands that can cater to that same feeling of alienation. But damn the image alone is so unappealing and unoriginal.