This is mainly in part to do with Major League Gaming through Doritos.
It's Halo because that is what draws the masses to MLG. Even the Halo MLG tournaments were broadcast on TV (G4). Plus everyone has heard of people like T-Squared, Str8 Rippin, and Final Boss.
Say all you want about consoles, but Xbox Live is the most organized online gaming you can get right now (yes, I know you have to pay for it, but that's not the point). I have a 360, a PS3, and a Wii, but I play games on my computer the most. The computer only has crappy ports for the most part, so any computer gamer can't really call themselves 'hardcore' because they're playing a second rate port.
You people need to get over yourselves and realize that as of right now, you're behind the times and stop complaining about it like grandpa complains about having to walk to school in the snow, uphill...both ways. You're not hardcore to him because you're not playing outside.