The Cube was used by my Horadrim brothers in many of their most complex rituals. When its potential became clear, they began to fear its power, the fools. They hid it away, even from me, and created a less powerful implement, the Horadric Cube, which they used for many years.
I never stopped looking for Kanai's Cube. Finally, I learned that they had entrusted it to a secret society of Barbarians. The last of these, Kanai, died in Baal's invasion of Sescheron. His spirit guards it still, and he is unlikely to give it to me. But I know someone who can impress even the mighty Kanai.