Because when thousands upon thousands of fireteams are reducing the amount of time it takes to breeze through PvE content after the Gjallarhorn was handed to them like candy, it may put the weapon's power in danger of being nerfed. Of course, this is speaking in the event that Xur sells it. I would certainly like it if people found it through RNG, but having it freely handed over to a million+ Guardians may have repercussions. Yes, this is a PvE issue and not PvP related, but Bungie has proven that they'll nerf things into the floor at the drop of a hat.