Why didn't he drop out after he finished a weak third in the New Hampshire primary?

ABC News:

Jon Huntsman will drop out of the Republican presidential race on Monday, a campaign spokesman told ABC News.

A source close to the Huntsman campaign said the former ambassador to China and Utah governor was "proud of the race that he ran" but "did not want to stand in the way" of rival Mitt Romney, the current front-runner for the Republican nomination.

Huntsman plans to endorse Romney at an 11 a.m. press conference Monday in Myrtle Beach, S.C.

After a disappointing third place finish in New Hampshire - a contest on which he had staked his candidacy - Huntsman vowed to fight on. In his concession speech in New Hampshire, he told his supporters: "I say third place is a ticket to ride, ladies and gentleman! Hello, South Carolina!"

But just six days from the South Carolina primary, Huntsman has said goodbye to the Palmetto state after all.

A source close to the Huntsman campaign said that Romney is "not currently" planning to attend Huntsman's Monday morning drop-out/endorsement announcement by the former Utah governor in South Carolina.

A Huntsman aide tells ABC News that the decision came in the wake of the results of the New Hampshire primary.

"He has been discussing with his family after they woke up after a successful evening in New Hampshire. They felt good about their performance in New Hampshire, but he and his family had a discussion and this is the decision they came to," the aide said. "At the end of the day he decided he did not want to hurt the best chance of beating Barack Obama and that's Mitt Romney. By continuing into South Carolina and Florida, that's what he would have been doing."

That last is nonsense. Huntsman was polling at about 5% in both states. The idea that his candidacy would have hurt Romney's campaign going forward is pure self flattery.

Huntsman is a serious man with serious ideas. But his temperamant and pragmatic politics was a bad fit for the average GOP voter this year. He could very well end up as a cabinet secretary in a Romney administration - perhaps even secretary of state.

But the presidency was never realistically possible.