I turn you over to Eunomia, who points out that Fred! just might be the luckiest man alive, next to Lou Gehrig:
Fred Thompson got out while the getting was good, before the full onslaught of the Bush Era, and so memories of his time in the Senate are a little more blurry and bound to be suffused with warm, nostalgic feelings for the good old days when real conservatives supposedly roamed the halls of the Senate. This obviously makes no sense. The reality is that Frist more naturally fills the “conservative gap” in the GOP presidential field than does Fred, but was so badly compromised by his time running the Senate under Bush and the subsequent loss of the Senate in ‘06 that he ceased to be viable. Thompson, had he remained in the Senate, would be in the exact same position politically. It is only because he happened to separate himself from the Senate GOP before it went careening to its doom that anyone takes him at all seriously. In terms of substance, he is actually a less compelling figure for conservatives in terms of his policy views than Bill Frist.
Maybe Fred Thompson is something the right just has to work out of its system. Then again, it took 6+ years to work past the "Bush" stage. If conservatism is going to move forward it can't go back to this good ol' boy southerner never meanin' no harm. The Republican party will lose with a retread, and that's just what Fred Thompson is.