When the humans that had evolved into the demonkind had arrived to hell, the knowledge they possessed of mana was poor and lacking.. However, since mana was still present in hell despite the mysterious lack of essence treasures, there still were people willing and able to comprehend its concepts. However, these cultivators only came to a later point in time.
The ancestors of the demonkind had been forced back into a corner, and once into hell, they found themselves in a hostile territory, from which they were barely able to survive. Their numbers were in the hundreds, and they faced extinction constantly through the many initial years.
Through hell's history, the first few hundred years were the period of time in which the interest in the mysterious stone found in the core of hell, was at its peak.. And even then, they had much more important things to worry about.. Things like survival, organization, and exploration.