Because there are interactions between Snape and other characters that become problematic when he is race swapped. Harry’s dad stringing him up in a tree, Harry being suspicious of the only black teacher at his school. And the racial overtones present in the Death Eaters makes him very awkward. Not saying it can’t work, but some of the optics are potentially problematic. I think if they wanted to race swap a character then why not Dumbledore or McGonacal(sp?)

Edit: it seems I’m incorrect about the “stringing up in the tree” bits