There are some pretty good debates possible in this question. Does one go with the "isolation" theory as /u/ZaschZogg puts forward? Or does one go along with scholars like Walter Rodney (and his intellectual descendants) who emphasize the "development of underdevelopment" that resulted from the Atlantic slave trade and the industrial revolution, as OP notes. Certainly, the fact that Europeans were not appreciably "ahead" in the 15th century (as the slave trade begins) seems to support the latter position, but in support of the other is the fact that Europeans had created a weapons gap vis a vis Africans partly by adapting and improving technology (like gunpowder weapons ) developed in China and the Islamic world.

My contribution to this debate is to suggest a deeper interrogation of what "behind" means. For example, there's ample evidence that sub-Saharan African sustainable agricultural techniques were every bit as advanced as those of Europe in the nineteenth century ( in fact, they were more sustainable ). Similarly, in terms of political systems sub-Saharan African societies had intensely interwoven nets between civil society and political operationality. Yet in terms of weaponry, industrial productivity, the ability to harness capital (funds and assets) to large-scale projects, and military organization Europeans were clearly more sophisticated. I would argue that those are the particular factors on which it would be useful to focus a question like this.

A big factor is that sub-saharan Africa ('Black Africa') was largely isolated until very late in history while most big innovations like animal husbandry or metal working spread slowly through Eurasia.

Other attempts to explain the difference in development could be that the climate of many parts of Africa, where the human once evolved were so ideal for his original way of life as hunter-gatherer that the humans who stayed in africa never had to develop any technology in the first place.

The answer to your question can be partially answered by asking a second question: Why did the Europeans have to wait until the 19th and 20th centuries before they scrambled for Africa? After all, the Europeans had much easier access to Africa than they did Asia or North America, but they only managed the colonization of small chunks of the African coast before the 19th century.

Why? Geography.

If you take a look at a physical map of Africa, you will see that most of the continent's interior is plateaued well above sea level. This gives Africa's topography an appearance similar to a cereal bowl turned upside down.

Because of its cereal bowl shape, the major rivers of Africa, which are very few in number compared to Europe or Asia, are very difficult to traverse by boat for long distances due to rapids and waterfalls. Except for the Nile, none of Africa's rivers allow unimpeded access from the coast to the continent's interior.

Africa is a massive continent. Without the ability to use river transportation as a means to colonize the Africa's interior, Europeans were stuck hanging on to coastal outposts until the invention of the railroad. For native Africans, the lack of navigable rivers with which to trade, communicate, and travel, made the development of large and long-lasting agricultural civilizations nearly impossible (except for Nile-based civilizations).

Several large civilizations did exist in Western Africa: Mali, Ghana, Songhai, etc. However, the climate of Western Africa is extremely unpredictable and rise and fall of several civilizations often lined up with periods of abundant rainfall followed by years of drought.

So why were Europeans able to conquer and colonize Africa? Luck. Africa did not an environment suitable for the development of societies capable of matching the military technology of Europeans.

As a final note, I must add that African cultures were not inferior in any way to European ones. Ranking societies is an ideological invention rather than a fact. African cultures developed in ways that suited their particular situation, just as European societies had. After all, Africans were able to create thriving societies on the interior of a continent that most Europeans considered inhospitable and unable of sustaining "civilization." I could easily call Europeans culturally backwards for thinking something so foolish!

Although he focuses on why Europe dominated the world, Jared Diamond's Guns, Germs, and Steel is a good book to read if you want some general information to help answer your question.