One would think that it was obvious, since he already had lost his Senate seat by a record number as well as had run under 2% for about 9 straight months. That, and the other impulse candidates like Cain, Bachmann, etc,.
The base has become too unstable to command any party. They have become a heard of cats.
If this cycle has taught us anything it is to remind us that the one sure fire way to clear a path for an establishment moderate is to encourage a scripture quoting social conservative (no need to be a small government conservative) to enter the race and stay in until the RINO wins. There is this persistent block of voters that will waste time and energy on silly candidates like Robertson, Huckabee and Santorum - candidates that have exactly zero chance to win a national general election, but can do well enough among the part of the GOP base that is made up of social conservatives to splinter the grass roots/activists and allow a clear path for the mushy moderate candidate.
If I were someone like Chris Christie or Jeb Bush, I'd be planning right now on which clearly unelectable general election social conservative to encourage to get in the GOP primaries for 2016 or 2020. Maybe pull Pat Robertson out of the 700 club studio? That is, if he isn't too busy explaining why the recent outbreak of tornado's is because people didn't pray enough. Believe it or not, Robertson actually did win some caucuses and over a million votes during his primary run in 1988. Oh sure, he could never win a general election, but he can split the grass roots and clear a path for the next establishment moderate in the next election cycle.
Santorum wasn’t vetted until very late in the game. He was never considered a real threat.