Because that's how you divide the country. The Democrats need to keep the country divided in order to gain or maintain power. I mean, imagine this: A country with no race issues. Now, if that ever happened, who would need Democrats around?
The thing is, who has been making race an issue?
Goldwater and his rejection of Civil Rights.
Nixon's Southern Strategy
Reagan's Welfare Queens and Young Bucks
Bush's Willie Horton Ad
Jesse Helm's White Hands Ad
Corkers "Playboy Mansion" ad.
Everything Trump says.
Here's the thing. The only way to get STUPID WHITE PEOPLE like you to vote for more plutocracy while your health insurance is getting cancelled by your boss while he takes Italian Vacations is to get you all angry that you are supposedly working hard, and some darkie at WalMart is getting a bit more than you because they qualify for benefits that you don't.
Or we could do something more pragmatic like have a voter test. You need to take a very simple test before you are allowed to vote. The questions can even be multiple choice.
Yeah, they tried that in the South. Oddly, they made the tests a lot harder when black folks tried to vote.