Because the person who wrote is either a successful troll, or clinically retarded. Neither are proper nouns or titles so neither should be capitalized. But for some reason the SJWs are obsessed with equity and this is attempt at trying to give them something that wasn't earned.

TLDR; We wuz kangz 'n sheeyit.

Because capitalized "Black" is a specific group and identity containing specific immutable physical and mental characteristics distinct from the rest of humanity by which you can identify them instantly, while white is a mere color identifier of which diverse and varied peoples of all nationalities, outlooks, behaviors, and norms exist within.

Or at least, that's what the grammar implies.