Of course I would have doubts about pulling causality out of something as complicated as this, but here is an interesting paper that talks about a few possible mechanisms [1]. The big ones it seems are:

- Inflexibility in when people may vote; The types of jobs people have access to has a racial bias. Black people tend to have jobs where they literally aren't allowed to leave during they which means they can't stop by the polling location during off hours. This can make polling places deal with larger peaks in predominantly black areas.

- Partisan resource allocation; predominantly black regions tend to lean democrat and so in republican areas they may receive fewer resources to deal with voting.

- Voter ID laws; These also have a racial lean and can clog up regions where they are enacted. Minorities also tend to have a more difficult time gaining access to IDs for voting and so people may have to come back multiple times or be turned away.

- State voting laws; Regions vary with who has access to early voting, vote by mail, etc. These tend to have a racial bias as well.

[1] https://www.nber.org/papers/w26487?sy=487