Because you don't need it. As Xyzzzy pointed out, at L19, you get the Ghost Spear Attack Series. It does far more damage, and has a much wider range, and also hits more enemies at once, compared to the Heuksal Spear Series (Dancing Demon). Sure, it's useful at first, but once you get to L19, it's obsolete.
Further down the road, you'll realize that, for a Glavier, the only two MUST-HAVE skills are Soul Departs Series, and your Ghost-Spear Series. Soul Departs if your bread-and-butter for both PvE, and PvP. Gives Stun in a 30% chance on the last book, which completely immobilizes the enemy. When fighting a Pure INT build at my Level, a Critical from a SSS, followed up by a Ghost Spear Mars will almost certainly kill the enemy. Those are the only two moves you need. By the time your Ghost Spear Series is done, your Soul Departs will have finished its cooldown time, and you can reuse it again, and hope for another Stun.
There is simply no reason to get the Heuksal Spear Series. It's useless. The usefulness of the Chains, even, are questionable, seeing as they run out your durability so quick.
SSS + Mars = All you need.
