Harriet Jacobs, a Play

I just finished reading "Harriet Jacobs: A Play", by Lydia Diamond.

Some years ago I took a "dialog workshop" led by the author. I was writing my first full-length play at the time, and using the class to test out material as I wrote. I was very productive for me, and she was an insightful and supportive teacher.

The play is based on Incidents In The Life Of A Slave Girl, which was written by Harriet Jacobs. This account seems to stand out, historically, because it contains an eye-witness woman's account of the sexual side of plantation life, a deeply taboo topic of the time, much whispered about, but not much written about in real detail.

The play's cast list includes the following, to me rather odd, stipulation: "It is imperative that all cast members are Black." It goes on to say that White characters are to be played by Black ensemble characters.

Well, that's an interesting way to present the play, but is it really imperative? No explanation is offered. Was it something learned in workshopping the play? Was it felt to be too inflammatory if people played characters of their own races?

What is it about the narrative
that makes this so imperative?
Is it a way to keep from inciting rage,
so that insight can take the stage?