Because of two things: 1)Europe and Asia has a more progressive bent when it comes to art. Metal is much more widely accepted in a situation like that, Try putting on a rock show in some parts of the USA, and you get Rev. Whoseorwhatsis from the Foggy Bottom Baptist Church screaming from the pulpit "That music is the work of the Devil. Satan himself I say! Jehova Rafa!" And announce a vigil in front of Glenn's house, knocking on his door to call on Glenn (or whoever the promoter is) to be "saved".
2) Usually, music lovers listen with their ears. That is what goes on in Europe, Asia and Latin America. Granted, the way the Radio licenses are, every band has a chance to be heard. So, if a person likes what they heard, they keep listening. However, Americans LOOK at music. Looks are what matters. If the singer/band looks "hot" or "cool", then so is the music. Do you think many teens and tweens will have a topless poster of IA Eklund or Tapio Wilska, and dream of being the next Mrs. Eklund or Wilska? Do you think teen and tween boys will get a rise out of looking at Angela Gossow or Lori Lewis? But, put up Justin, Britney, Gwen, or Marc Anthony, and watch the teens and tweens go ga-ga.
That is the reality.