How race hurt Cory Booker — and the rest of 2020's diverse field

The question that Democrats are asking now, on the eve of the first all-white debate of the cycle, is why? Why did two high-profile black senators (Cory Booker and Kamala Harris) drop out before Iowa? Why didn’t Julián Castro, America’s most prominent Latino politician, ever crack 2 percent in the national polls? Why are all four of the remaining front-runners white? Is race the reason?