the story they have been saying is it was MLK. He said to her in 67 after the first season that she had to stay. All other roles were black stereotypical roles and positions, but Uhura was showing the future where you could do any role
One of my favorite bits of trivia from that show was that they showed the first interracial kiss on TV and got it past the network censors because it was a show about the distant future. However, in the South, the networks didn't care and insisted on a chaste kiss between Kirk and Uhura. Shatner intentionally screwed up every single take they tried to do of the toned down kiss. He'd make weird faces, look directly into the camera, etc. Just wouldn't let them get the footage to even splice something together, so they were forced to run it around the nation with the full on kiss.