Duncan Hunter just plain can't win for the same reason Paul won't: House members that are not in leadership positions (Speaker or Minority Leader) don't have a large enough base to run for the Presidency, and they don't have executive experience to run on.
If Hunter runs and wins a Senate seat (fat chance in CA) or gets elected Governor and can somehow corral the Demo legislature, that will make him a threat.
For example, of these four, identify who's voting record each is.
A.
B.
C.
D.
Answer: A. Fred Thompson, B. Rudy Guiliani, C. Ron Paul, D. Joe Lieberman - Source: http://www.ontheissues.org C.