[M]aybe, as some have said, it's just blind rage revenge against Mitt Romney, even if by staying in he's actually helping Mitt. After all, Newt is a great snake oil salesman, but not a particularly good strategist, whatever he constantly says about himself.
Massie is on the same page:
[I]f nothing else, Newt's kamikaze assault on Romney won't make him the nominee even were it to be successful. That's one of problems with kamikaze missions, you see. So the only people who really beneift from this are Newt's other rivals and, more importantly, Barack Obama.