...which needs more to explain it, such as (if true): "the diversity originating in Africa did not migrate out of Africa, and the gene lines that did leave Africa did not develop as much diversity as had existed in the ancestral gene pool, despite facing far more diversity in climate than exists in Africa".

And then I would again ask of the latter, "what? Why not?"

Ultimately: Why did Africa stimulate so much gene pool diversity? It seems more homogenous in environments than the rest of the world that Homo Sapiens emigrated to.

And there would have been cross-breeding between African subpopulations. Environmental barriers in sub-Saharan Africa are minimal in terms of gene movement, aren't they?

Existence, not history. History requires writing.