Because Bungie screwed up with the ToO and they know it. The barrier to entry to the ToO is too high, people aren't going back and trying it because it is too punishing. The elimination mode would need to be permanent to give people the ability to practice and get used to the mode, but it isn't. The ToO would need skill based match-making so that people don't get pub-stomped and have a modicum of success to continue to engage in the mode to get better, but there isn't.