Why are tons of metal bands not doing tours in North America?

OK. So I've been dying to go see a metal concert. slipknot, disturbed, dragonforce, and black tide are all great bands and they played a concert near me but I heard they are all bad in concert. So I checked out many other bands and theres no tours in Southern California for all of 2008!? Korn, Slayer, Godsmack, Metallica, Iron Maiden, and Black Label Society are doing tours mostly in Europe, Italy, and Spain and not North America! WHY? Also I'd do anything to see a S.O.A.D concert but they arent together. Scars on Broadway is a good band but they are also in Europe doing tours there....