Because a ton of players are using this "bug" to their own benefit, and Bungie is leaving them be. What about those of us who couldn't beat the mission or even run it? We lose out on a chance of getting all our special weapons to 300. It's a daily mission that rotates. This isn't another Three of Coins scenario. People can't farm the boss without exerting a ton of effort and actually having it unlocked in the first place. Why bother changing it?