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Pawlenty, Daniels, Christie even Huckabee and Bachmann. The "why" is simple - none of them are afraid of the media, to include the hostile media. Palin, OTOH, never goes anywhere to speak to anyone that isn't named Glenn Beck, Sean Hannity or her favorite, Greta Van Sustern. If you're afraid of a journalist, you're not qualified to run the country.
Marco Rubio hasn't been in the Senate for more than 6-months, and yet, he walked into Meet the Press on Sunday morning and OWNED David Gregory. He was clear, concise and compelling. Palin, on her best day, doesn't reach any of those measures.
When Palin appears on a program that isn't a Fox property, and does half as well as the freshman senator from FL, maybe she'll merit some consideration, but until then - no thanks.