Because Google no longer just searches for literal strings of text, ranked by links.
They now infer your meaning and find results that match your intent -- even if the exact search string doesn't exist in the search results.
For more on this, read about Hinton's work on "thought vectors" [1].
So if Google can't determine the meaning of African American language as well as it can standard English, then that content is likely to be less visible in search results.
[1] https://www.theguardian.com/science/2015/may/21/google-a-ste...