Why doesn’t Ron Paul just quit?
Ron Paul is the head of a Libertarian/Populist cult following. It is a religion and he is their Pope.
If Newt drops out, Paul will likely get most of Newt's remaining voters with an even split going to Romney and Santorum. Most of Newt's voters who have already left, mostly in favor of Romney, were those who were voting for who they thought had the best chance of beating Obama.
The remaining die hards for Newt are Anybody but Romney hold outs, who refuse to support Santorum. They will favor Paul, who is almost a guarantee to stay in until the Convention.