I also heard a quote from Simon that went something like: I always knew there were a lot of great singers in America, and this year proved it. He said this year was the best talent collection for the show they have seen.
American Idol is not just about "can you sing". It is about whether you can be a pop star. Poise, appearance, voice, etc will all figure into this.
Mama Cass would not be in the running. Neither would Joey Ramone or Bob Dylan. But all left big footprints on the music world.
Even talented singers are turned away and take offense when they are told "you can sing but...".
After enough seasons, it would appear that those who ARE right for the show know how to present themselves in the audition.