The most likely reason was simply that Africa, from the point of view of 19th century Europe, was largely unknown, at a time when Europeans believed that they knew the rest of the world.

I'm pretty sure the chief meaning was "dark" in the sense of "unknown." The opening words of an 1890 book entitled "Stanley's Adventures in the Wilds of Africa" make this clear: "For centuries Africa has been 'the dark continent' of our globe. The sea-washed edges of this immense tract have been known time immemorial. Egypt, at its northeastern corner, is the oldest of the governments of the earth; while the nations skirting the Red and the Mediterranean seas were actors in the earliest recorded history. But Africa as a whole has been an unknown land." My wife went to school in the 1950s in a small schoolhouse, and the big map of the world on the wall had been published in the 1930s and still had parts of Africa that were labeled "unexplored." In the days when maps were made by people hiking in with surveying equipment, Africa was one of the last inhabited places to be explored and mapped. Of course the whole idea of "exploration" reflects a European view of the world, and an imperialistic attitude. "Explored" means "explored by Europeans." There's no doubt in my mind that the phrase "the dark continent" also was influenced by the skin color of the people living there. Before and during World War II, military necessities resulted in advances in aerial photography and many blank areas on the maps were filled in from aerial photographs... and then satellites finished the work. The idea of a map with blank spaces on it, or the very word "unexplored," sounds quaint and old-fashioned today.

"Hunger" "Poverty" "Illness" "Racial Discrimination" "Black Slave" "Great Nature" "Lion" "Emergency Life" etc. ... .... Maybe it is such a place. But.... Rather, it was much more prosperous than Europe. In terms of being absorbed in alchemy, astrology, witch trials, etc., the Europeans did not sink to Africa at all. In Europe of the 15th century, superstitious Christians gathered and danced in cemetery often in the precincts of the church believing that they could escape from the plague. At the same time in Germany, the average of two "witches" a day was sentenced to burning. Even though I believed in such bullshit, the Europeans were proud of themselves being noble, sensitive and rich. The image of Africa, far from Europe and having a different culture, is merely made by such people.