Why does Conrad delay the real-time appearance of Kurtz?

When Joseph Conrad wrote Heart of Darkness, his scathing indictment of European imperialism and the exploitation of indigenous peoples it entailed, he clearly hoped to illuminate to the extent possible the physical and emotional toll exacted on conqueror and conquered alike of those practices. The character of Kurtz, a figure of almost mythological proportions as Conrad’s narrative progresses, finally makes his appearance near the end of Heart of Darkness because Conrad wanted first to illustrate the depths of depravity, racism, and greed that created Kurtz.
As Conrad’s protagonist, Marlow, embarks on his journey into the interior of the Congolese jungle, he encounters individuals along the way who progressively stoke his interest in and eventual obsession with the mysterious station agent. This interest in Kurtz, however, needs time to incubate. Marlow initially displays a distinct lack of interest in Kurtz, evident in the following conversation he has with the accountant:

One day he remarked, without lifting his head, "In the interior you will no doubt meet Mr. Kurtz." On my asking who Mr. Kurtz was, he said he was a first-class agent; and seeing my disappointment at this information, he added slowly, laying down his pen, "He is a very remarkable person."

This lack of interest in Kurtz continues as the extraordinarily productive agent continues to be the subject of admiring conversation while Marlow’s boat works its way down the river. Note in the following passage Marlow’s growing disdain for what he believes to be an increasingly ubiquitous, if unseen, presence:

There were rumors that a very important station was in jeopardy, and its chief, Mr. Kurtz, was ill. Hoped it was not true. Mr. Kurtz was... I felt weary and irritable. Hang Kurtz, I thought.

As Marlow proceeds on his journey, however, his initial lack of interest in Kurtz grows into a sense of fascination and obsession. This fascination with the figure of Kurtz is a product of the surrealistic and horrific stories Marlow hears and the scenes he personally observes. The deeper into the jungle his boat takes him, the darker and more mentally exhausting the experience. At one point, Marlow is told of another European, a Swede, who joined with innumerable others to seek fortune at the expense of the land and the peoples who inhabit this forbidding terrain:

"It is funny what some people will do for a few francs a month. I wonder what becomes of that kind when it goes upcountry?" I said to him I expected to see that soon. 'So-o-o!' he exclaimed. He shuffled athwart, keeping one eye ahead vigilantly. "Don't be too sure," he continued. "The other day I took up a man who hanged himself on the road. He was a Swede, too." "Hanged himself! Why, in God's name?" I cried. He kept on looking out watchfully. "Who knows? The sun too much for him, or the country perhaps."

Combine these tales of the mental toll taken on those who come to exploit the region’s riches with the soul-deadening appearances of the native Africans forced into brutal labor under dismal conditions and the prospect of encountering a fellow European at the end of line, who, it is revealed, has come to view himself as a god among savages, becomes too much to ignore. Kurtz is a rising star in the Company, so proficient is he at accumulating ivory on behalf of his employer, but one who has grown in stature at the expense of his own sanity. The only way Conrad could methodically illustrate Kurtz’s road to madness was through the observations and education of Marlow, the man who will become caretaker of Kurtz’s legacy. Heart of Darkness is not about Kurtz; rather, it is about the journey of a British seaman who comes to understand while traversing a long river into the Congolese jungle how morally corrupting is the policy of colonialism.