There was a time when the other lesser candidates started dropping out, that an anti-Romney could have been successful. The problem is, Newt's ego and Santorum's obtuseness prevented either one of them from doing the right thing for the country, stepping aside and letting there be one conservative candidate to rally behind. Now, they have effectively split the conservative vote between themselves, and left Romney's campaign of attrition to march to a slow but steady victory.
The idea that either of these knuckleheads are going to be rewarded with anything other than a kick in the pants by the Rough Boot of Historical Footnotehood is laughable. Mittens was on Leno, joking that he might make Santorum his press secretary one day. Soon, Rick will look back on the time where people bothered to make him the butt of their jokes as 'the good old days'.
Sadly, the joke is on the rest of us. By their own reckless ambition, Rick and Newt have allowed Mitt to advance unchecked on to the nomination. If it comes down to a brokered convention, neither of them will be rewarded for it.
The problem was that neither had any reason to drop out. The voters never really liked Newt and Santorum was never really a good candidate.
Santorum thought "I can't drop out for this guy, people just don't like him." And Newt thought "I can't drop out for this guy, he's just not a good candidate."
The problem was, they were both right.
What makes you think anyone else would be interested? Granted if Jeb Bush threw his name in the hat, he would win it, but it's all dependent on him deciding to run. Someone who has a serious chance at winning the nomination through the normal process in future years might not want to throw a Hail Mary pass on an unprecedented attempt to take down a sitting President by entering the race with only 2 months to go.