The answer, of course, is because white people are the default race and they really didn't feel like trying. Ari Handel takes the stance that race doesn't matter to the story, therefore only white people are depicted. POC would be distracting, apparently. Also, he adds that putting in too many POC makes a movie too much like Star Trek.
"From the beginning, we were concerned about casting, the issue of race. What we realized is that this story is functioning at the level of myth, and as a mythical story, the race of the individuals doesn't matter. They're supposed to be stand-ins for all people. Either you end up with a Bennetton ad or the crew of the Starship Enterprise."
This really annoys me because obviously, race DID matter. The makers of this film made a decision to cast only white people. If race really didn't matter they wouldn't have made that decision in the first place. If race is truly not an issue, the race of the extras should be about proportional to the racial mix of extras who applied. Using white people as the default race, stand ins for all if humanity, doesn't side-step the race issue, it is a racially charged decision.