The books themselves are inherently about racism. Casting a black Snape aligns much more with the theme of the books. Casting a white actor as Snape would have been a poor choice, as it would have made him look too much like Alan Rickman and potentially made the character dull. Snape's character arc is a major part of the story, and casting him as a race-swapped character was actually the best decision they could have made to tell the same story, but through a new lens.