I think possibly because without a protagonist-antagonist binary, you only have yourself. African blacks lack the legacy issues and come into a "clean" slate, i.e. that binary dynamic doesn't exist for them. It's certainly not a simple issue.

Depends which parts of Africa they are from.

I'm not an expert in this area, by any means. And I certainly can't speak for these people. But, I imagine on average that the people who choose/are able to immigrate here have an above average work ethic, skill set, or access to resources. There are probably many people who never have the chance to start a life here. Its not a direct comparison.