The soul was the source of all supernatural abilities in the world of Eich.
The entire world was the container as souls went through their cycle.
Heaven's Pillars and the River Styx were the core components of the Cycle of Reincarnation. However, all of the Heaven's Pillars in the various Gods' Divine Kingdoms were far inferior to the River Styx in capability. The Heaven's Pillars only had to transport the soul to the Divine Kingdoms, while the River Styx went through all the countless dimensions.
The foundational reason for this had to do with the difference between Order and Chaos. Since the Heaven's Pillars were part of Order, they naturally wouldn't change. Only transporting the souls would be impossible to help the entire Cycle of Reincarnation make progress.
Thus, the River Styx which had its own natural cycle was far more useful to the world than the Heaven's Pillars. That was why even the new dimension of Hell had the River Styx's presence.
