Because of the high profitability and difficult work of hunting ivory in the jungle, men who could command respect and power were well-suited to lead hunting teams. However, Kurtz's original intent was not simply to be a successful ivory hunter, but to win the heart of a woman, whose family rejected him because he was poor.
I had heard that her engagement with Kurtz had been disapproved by her people... He had given me some reason to infer that it was his impatience of comparative poverty that drove him out there.
(Conrad, Heart of Darkness, eNotes eText)
Determined to prove them wrong, he entered the jungle and became very successful, bringing in "as much ivory as all the others put together." However, the power he commands alters his mind and eliminates his conscience, and although he continues to send ivory back, he also sets himself up as a god to the natives. It is his failure to realize his original goal that leaves him with nothing to strive for except his own personal power and glory. Had he remained sane and become wealthy, he might have been able to leave the jungle and return to civilization.