For the same reasons other people are. Religion, tradition, or actual conviction. In a healthy political system, African Americans would be about 45% Democratic, 30% Republican and 25% independent. The Democratic contingent would be similar to the current African American Democrats except younger, more liberal on social issues, and more progressive. Their top issues would be things like BLM, criminal justice reform, affirmative action, ending the drug war, and economic populism. The African American Republican contingent would be older, religiously conservative, and their top issues would be things like restoring black families, the divorce rate, pro-life, school choice, black business, and anti-immigration.
However we don't have a healthy political system because one major party is virulently anti-anybody who isn't white (and the other is starting to turn a blind eye to anti-whiteness), so all the African Americans vote Democratic and end up favoring establishment candidates.
We've basically seen in majority Black cities the Black vote split into two factions:
One that is fiscally neo-liberal, socially liberal, technocratic, pro-charter school
and another that's basically non-ideological and supports organized corruption
there's a third faction of hyper woke college students but they aren't big enough to win on their own and generally lob on to one of the other factions, usually the one out of power
