I think the reason just boils down to "It looks stupid". Metal is an image and attitude, whether people want to admit or not. That is why you hear people talk about Metallica and their hair cuts, Anthrax with Joey's frizzed out poofy hair and so on and so on. Just about every band has an image and that is why you hear about it so frequently.
Disturbed-Hot Topic Core
Korn-Baggy pants and eye brow piercings
Corrosion- Good ol Southern boys
Clutch-Dudes that look like they could work in a tool and die plant
Slipknot- Masked Gonads
S.O.A.D.- Douchebags
Satyricon- White face paint to look like they haven't left their basement in 17 years.
S/T- L.A. thugs
With that being said, I don't fit the mold of an average metal head. I wear dress clothes and suits(when needed). I don't care what I look like. All I know is I like the music I listen to and don't give a shit if I look metal enough or not.