Would it be too flip to say that this question is more than a matter of black and white? I think there's an underlying assumption here that whiteness and blackness are two distinct and unified categories with a consistent historical meaning. In reality, both are constructed differently in different eras and in different places. To look back at the slave trade in the seventeenth or eighteenth century with present-day lenses of who counts as white and who counts as black imposes contemporary categories on historical actors and obscures the complexity of the story.

It's often argued that the definition of whiteness in the United States has become more inclusive over the past century––such that Italians, Jews, and other groups who may not have been considered white in the past became white later on. It's also true, however, that the idea of a unified African blackness––and the naturalization of a racial solidarity based on that blackness––is a later historical imposition. No one seems to argue that soldiers in wars between France and England or between Germany and Russia betray their whiteness. Yet the question of how African people could be willing to betray their blackness by engaging in wars (and then trading slaves from those wars) seems to arise continually. Africans fought wars with one another because they constituted (and constitute) many different nations, just as Europeans did.

I think the important distinction to be made here is between responsibility and complicity. Western nations (at least some of them) bear responsibility for the slave trade because they created, engineered, and profited from a global economic system that moved African people all over the New World to create wealth for the West. That economic system was enormously powerful, and without it the slave trade wouldn't have been possible.

It could be argued that African groups who traded slaves for use in the slave trade (typically from enemy nations) were complicit in that economic system, and I think that argument is convincing. But the problem is one of scale. Your question is why historians don't apportion equal blame to people who were complicit in the slave trade as we do to those who ran it––I think the answer is pretty clearly that Western nations played a much larger role in the origin and continuation of the slave trade than did any other group. That's not to excuse people who participated in the slave trade at a lower level, but I think it's important not to draw a false equivalency here. I hope that answers your question.

While the role of Africans in the Transatlantic slave trade is important, it's worth noting that they operated in a system of supply and demand. There are numerous examples of African authorities taking quite drastic steps to increase the numbers of slaves they were 'making' when they realised just how many slaves Europeans were prepared to buy, and for how high a price. There are many cases of African rulers applying slavery as a punishment for extremely petty crimes, or starting wars specifically in order to take slaves. So, while the African rulers had agency, in a sense the European traders' influence was felt right at the very beginning of the slave trade, e.g. when the person in Africa was originally enslaved.