EXPLANATION: "Noah" Screenwriter Ari Handel Addresses the Reason For An All White Cast: To Represent "Stand-ins for All People" (Diane Cho, Complex).
I raised this issue in passing in my review of the movie, but I didn't really think through the implications until later. The problem, of course, is that by making everyone white before the Flood, the film opens the door to the old racist meme that dark skin color is the curse of Ham. Now to be absolutely clear, the movie does not include the curse of Ham (although Ham does go into self-imposed exile) and I am not for a moment suggesting that the writers intended any such implication. Still, I do not find their explanation satisfactory. Myths do matter, and the problem could easily have been solved with a little diversification of the cast.
Much more on Noah here and links.