The answer is very simple and straightforward: Because Democrats buy voters. That's what they do. They pretty much tell you "vote for me and i will give you money". Not "vote for me and i'll try to create the right conditions for you to make money". It's "vote for me and i will give you money". Forgiving the student loans is a great example of it. They pretty much just bought the entire academic demography by saying that.
Given african american's poverty rates, they're very easy to buy. It's like buying a hooker. You just give them money and they're yours. A good portion of them are poor, so you offer them money and they will like you. It's a simple, easy to understand, appealing contract. "Vote for me and you will get loads of free stuff, just because of your color". Does it work? No. Just like winning the lottery often doesn't work. But it's like giving chocolate to a dog. He will eat it because it tastes good and he only cares about having their immediate, most primitive desires fulfilled.
And that's pretty much it. Funny enough, the black community has a lot more in common with conservative values than liberal values. Take away the desire for easy money, and african americans are more conservative than most republicans.
I mean, wtf does the black community have in common with the LGBTOIKEORIE crowd? Go be a transwhatever in a black neighborhood and tell me how tolerant they are.