The point is: when people resist more open casting, in a lot of cases they don't really think they have any particular prejudice. They are acting upon a vague feeling. It feels "right" to them to have a white man as the hero because those are the heroes they grew up with, that inspired them to then create their own stories. It feels "normal." Until someone comes along, challenges that norm -- and they realize, 'Hey, this works, too!'
Science fiction fans might want to note two series that recently premiered: Dark Matter and Killjoys. Killjoys is created by Michelle Lovretta, who created the popular fantasy series, The Lost Girl. While Dark Matter comes from creators who oversaw the Canadian-made StarGate TV series.