Why? Because he's scared, and he's a racist asshole.
"Of course, former President Donald Trump’s campaigns are always nasty work; that’s to be expected from a loud-mouthed, racist, sexist, petty, congenital liar of a candidate. But Trump has always singled out Black women for his most denigrating abuse."
https://www.msnbc.com/opinion/msnbc-opinion/trump-kamala-harris-attacks-racism-sexism-rcna163313
