Because she has the idiot bravado of the talk show regular.
I write this 2/3rds of the way into her "debate" with Chris Coons in Newark, Delaware.*
Sarah Palin was wounded by Katie Couric and Charlie Gibson in their 2008 interviews because she seemed at some level aware of what she didn't know. She was obviously uncomfortable with Couric's "What newspapers, specifically, do you read?" question because she sensed that the topic held perils. She acted the way I would if questioned about, say, opera. ("Which ones do I like? All of them! They're so great. All those notes!")
In this debate tonight, O'Donnell has not seemed uncomfortable for one second** -- even in her most obvious dodge, about whether she really thinks evolution is a "myth." The difference is, she is a talk show regular. Among the many things wrong with talking-head gab shows, which have proliferated/ metastasized in the past generation -- they're cheap to produce, they fill air time, they make journalists into celebrities, they suit the increasing political niche-ization of cable networks -- is that they reward an affect of breezy confidence on all topics and penalize admissions of complexity, of ignorance on a specific topic, or of the need for time to think.