First of all, we need to clear one thing, slavery wasn't solely focused on just one race, for example it also existed in Europe from middle ages till mid 19 century in a shape of serfdom, where more wealthy white people basically enslaved the poor, similar things were happening in Middle East, eastern Asia, etc. even in 20th century.
Now considering Africa, it just unfortunately happened that in those harsh days of the past (when basically no human rights existed) Europeans were much more technologically advanced than African (and native American) people, so they were able to impose their will on them, ones were basically ethnically cleansed from their territories (native Americans) and the others were enslaved (African people).
It's also kinda interesting to see how in latin part of Americas indigenous people weren't treated so harshly and haven't been ethnically cleansed to the extent that happened in Anglo-American part of the continent, in fact Spanish and Portuguese colonists basically just simply mixed with them.
Now, the key issue in strictly capitalist driven USA is that if you're down in food chain (for example like most of minority people, but recently many white people also suffer the same fate with disappearance of the middle class), it's extremely hard to bring yourself up other than through sport scholarships, and therefore it's extremely hard to expect the equality when you have most people living in ghettos, hoods, poor people areas, where there's little hope for success.
For example, many current race issues would vanquish if USA would be willing to implement some sort of Canadian/Scandinavian social, educational and health systems, which would then help the underprivileged (no matter the race) to achieve some sort of equality.
But I guess in USA it's more important to spend trillions of dollars for army and weapons (so you can accomplish some sort of world wide dominance), rather than focus on well being of your citiens.