Quote from AAAintheBeltway:
Fred Thompson is a very smart guy. He was a high level Washington lawyer, a U. S. Senator and an actor. He is a southern conservative who can make an articulate and compelling case for conservative principles. He projects an image of mature judgment, integrity and strength that none of the other candidates can match.
The other Republican candidates have obvious flaws that will either impede their ability to motivate the Republican base, eg Rudy, McCain and Romney, or appeal to the public at large, eg Newt. Thompson can do both. He has solid conservative credentials and can appeal to the independent and swing voters. Plus, never underestimate the value of celebrity in today's culture.