They use Xurs inventory to entice people back into playing the game, since most of the dedicated players are bored with PoE and HoW in general, play time for people has dropped like a ton of bricks. So now they are selling the Gjallarhorn, to artificially increase play time back to players who didn't have Gjallarhorn back to normal, people are going to grind their asses off to level up and use Ghorn. They did the same when they sold Ice Breaker in TDB.