Since this was a book not only about his own father's real experience but also the real experiences of some six million murder victims, it was very important to him that he not further the very stigma that had been used to justify those murders.
Zootopia is a recent movie which also uses animal relationships as a metaphor for prejudice, but its conception was very, very different. The filmmakers started with the idea of a movie like Disney's Robin Hood, whereby they told a story with a bunch of charming talking animals. (Which is probably why main character Nick Wilde looks exactly like Robin Hood). They wanted to create a modern world wherein animals use iPhones to listen to familiar songs and drive cars we also might want to drive. A world where we could see ourselves as the animals. Once they had the idea for this city, they realized that they could also say something about bias, prejudice, and racism. That's why they inserted familiar stereotypes like only bunnies being "allowed" to use the word "cute" (just like the N-Word!), or it being rude to rub a sheep's wool without asking (just like with black people's hair!), or people being traumatized by childhood bullying based on their appearance.
And I'm pretty sure this is why the whole messages goes to hell.