Heart of Praxic Fire is a symbol. A symbol of Hope. A symbol of Love. A symbol of Passion. One of the last refuges of gear that's not flooded the market, along with Fatebringer/Thunderlord/Hardlight. If Xur sold any of these items, what would you continue to do the Nightfall for? Grind out the pointless Atheon or Templar fight for? Run countless strikes hoping for a chance to see a stupid dreg running around with it? Save a the games life Xur, keep selling void fang vestments. HoPF isn't that good anyway.