Taking Ron Paul out of the equation, Santorum is the only candidate that has any semblance of an "everyman" sort of quality about him.
As I mentioned in the other thread, Romeny couldn't possibly be any better of an embodiment of the stereotypical rich a-hole antagonist from the movies if he tried, and calling him a conservative based on his record is a real stretch (granted there's only so much one can do in Massachusetts, but it is what it is). Newt's whole campaign is based purely on angry and talking points, and Mitt's superPAC ads revealed the fact that Newt is the definitive Washington insider, which turns people off.
Santorum comes the closest to being the standard Republican neoconservative of the three, but he's still trailing Obama by double digits in nearly all of the major polls, and getting any real momentum with swing voters would be miraculous.