Lol, Disturbed make the music they want to make, and that's it. Just because it isn't a wall of noise, there isn't uncontrollable screaming and a tuning dropped so far you cant hear it, doesn't mean it isn't metal. They are a melodic metal band (yes such things exist).
If you class Disturbed mainstream, then you're just lumping together a lot of bands there which you can hear in their influences, stuff like Pantera and Metallica. Mainstream sounds like everything else, yet when I listen to a Disturbed song, it always sounds like Disturbed, even when they do covers. Really you don't get mainstream metal, you get mainstream Pop, and Hip Hop.