It doesn't? The first step into clarifying all this would be to look at the buyer's balance: investment, costs and benefits. The price of a Black slave on the market, the costs for lodging/feeding him and how much he produced. Had You looked for those data?
Sure, but such cost-benefit analyses (a horrible term to use w.r.t. to humans btw), would apply to both local slaves and imported African slaves. So the point remains: why would an African slave be economically better (i.e. cheaper) than a local slave. All the costs you mention (lodging, feeding, etc.) apply to locals just as much as to Africans, so it provides no basis for differentiation.
That's been my question from the start, and I continue to believe that there's no sound economic argument why African slaves would make more sense. The most interesting response I've received so far has been user @Keep's point that enslaving local Indians was banned, i.e. made illegal, and that it was the Jesuits (i.e. a European religio-political organization) that pushed for this ban. To me, this makes it pretty clear that there was something else going on other than financial/economic considerations.