I wondered about that too at the time, but the polling data was saying that if Gingrich had pulled out his supporters would not necessarily have gone to Santorum. If they went to Mitt instead, then Mitt would have gained votes that would otherwise have gone to Newt.
There are people who just don’t like Santorum.
There are people who just don’t like Newt.
Partly because of the cult of personalities. Partly because of bad blood between the supporters of each guy. Partly because of the personal histories/stands of each candidate.
If any of those things weighs more than principle in the minds of voters, we’ll be stuck with Romney. Because of the priorities of the voters.
As I said they valued defeating Santorum over Romney. The result is what we see now.