My take on it is that they gave the award to Green Book because it contextualizes the civil rights story in a metaphor of unity. The story of the Klan may have seemed too out of date to be relevant to today, even though it was more recent than the setting for Green Book.
It makes me wonder how many people today actually know what the Green Book was all about. I watched a documentary on it years ago and it opened my eyes to something I didn't know about before.
The Green Book was real and it was a lifeline to thousands of African-American travellers or almost 3 decades. The Green Book was vital for security and even survival.
Here's a short video on the story behind Victor Hugo Green's 'Green Book'.

