Not sure if it's been said, but to be blunt, because gamers (or at least, people on [gaming] forums) tend to be smarter than the people you meet in real-life.
"Wow, that's an arrogant thing to say!"
Well...
The thing is that metal isn't a very accessible genre. You will surely know people who say "it's just a bunch of noise" or "I can scream in a microphone really loud too". Yes you could, but it wouldn't be metal. This mostly boils down to the point that "most people" are stupid. They don't care about music; the 'music' they want is a tune they can hum along with after hearing the song once. AKA, accessible music. Metal generally tends to be about more serious encounters in life and emotions/experiences "most people" never have. It's sad, but they can't relate to something that has actual depth.
"That's all fine and well, but how does that tie in with internet gaming forums?"
It's simple, really. I'm not saying there aren't stupid people in games, it's just that the people who have the drive to actually go to forums like this one and talk about more meaningful stuff are also usually the smarter type of folks, ergo, they take metal over pop. Which basically brings me back to what I said in the first place - especially on the internet, if you look for smart people, you're going to find them.
That's just part of my theory, anyway. At the risk of sounding like a conspiracy freak; once you realize some things, you'll see it's all connected.
As a disclaimer, if you think I'm an arrogant elitist sonnova - I am.