Listen this is likely a combination of a lot of things.
1st: Activision. They've already proven to be spoiled children as far Brutal Legend is concerned. So when they shell out the rights to make a Guitar Hero: Metallica game of course they're going to put writing into the contract to make sure other games aren't going to get Metallica anytime soon.
2nd: Metallica. Say what you want about Meallica, but they're successful, and I'm willing to bet the rights to drop their music in your video game does not come cheaply. So if you're trying to make a game and you're already shelling out to Lemmy, Lita Ford, Ozzy, Jack Black to Voice Act, and then have to pay for the rights for some classic metal. Then when you think about how many metal heads feel about Metallica (if you haven't figured out how they feel by reading this thread there is no hope for you)it likely isn't worth the cost when you can put another Sabbath or Judas Priest song in the game.