Because, and this isn't trying to be racist at all, but most people who are black like Rap/hip hop, and most white people like metal music. Don't believe me, attend a concert for each, you'll see a larger number of white people at metal and black at hip hop. Most people who are into hip hop are also more sexually active, because watch the dances. Hip hop and rap is a lot about the erotic dances between people, where metal is about beating the shit out of each other. Thus, gamers.
Also, when you play games, you're killing things or fighting things in almost every video game. Metal music is perfect for those games, where as playing hip hop isn't as, battlely feeling. I'd rather kill my opponent while listening to a head banging solo than listening to the same beat repeated over and over and some guy rich enough to put it's entire audience to shame acting like he knows what it's like to be a gansta in the hood livin' off scraps and fighting for everything.
I like any kind of music except Rap, the falseness and just the obnoxiousness of the beat turn me off that (as well as techno)