Because when Santorum endorsed Specter in 2004 at the behest of his president and party, he committed an egregious unforgiveable error and needed to be entirely removed from consideration from the outset of the campaign.
When Newt endorsed Scozzafava in 2009, sat on the couch with Nancy, and cheated on his wives, he just committed small lapses in judgment for which he's apologized and we should just "move on."
What is wrong with Newt is more than just momentary lapses of Judgement. It is an indication that he is not a "true believer" and that he has become too much the poll driven politician. I recall the media spanking him hard with "the Gingrich that stole Christmas" headline, and the "Whiny baby shut down the government because he didn't get his way on Air-Force one" headline among others.
Apart from his complete tone-deafness regarding enemy media, he has done too many things that no true believer would have even contemplated. The media will KILL Newt if he should get the nomination, and I have little doubt he will back-stab us should he win. (As did George H.W. and to a lesser extent George W. Bush)