Because they don't look like typical punk little kids or have really stupid haircuts. The key to getting into the mainstream is really stupid haircuts and looking like punk kids. See AVENGED SEVENFOLD.
Be happy Sevendust isn't into the mainstream. If they are doing well, getting their hits and their fanbase isn't being completely overrun by moronic kids who think they are the gratest rox band alive doods, be thrilled.
Being mainstream shouldn't be a big deal for metal bands anymore. Being successful and being known around in the metal community as being good or very good should be more important than what Rolling Stone, MTV and Fuse think. If they got into the mainstream, you'd get tired of them very fast even if you are a fan.