Obama in the MCU

[SirStrangefolk]

Why couldn't either Chiwetel Ejiofor or Djimon Hounsou appear in Black Panther?

Alfre Woodard appeared in Captain America: Civil War because the CA:CW people didn't know that she'd already been cast in Luke Cage.

Because Mordo doesn't have anything to do with Black Panther and Korath is a dead body in space who has even less to do with it. Ad you say, it is likely the casting of Alfre Woodard was due to a lack of communication, or at least because the movies and shows are considered somewhat seperate.