How do you make it through the first round of American Idol?

I'm going to audition for American Idol this week and I was wondering, what do you have to do to make it through the first round of producers. I heard that 10,000 people will show up and only a few hundred are selected from that day, but why? Are they looking for a certain look, or an interesting back story ( my past is very complicated and I feel it's something that would appeal to them), or what? How come amazing singers like Hilary Scott (lead singer of antebellum ) Colbie Caillat, and Amber Riley (Mercedes from Glee) didn't make it though this round. If you have any interesting audition stories or insights about the producers and what they are looking for, please let me know.