Politics 101 - suspend campaign. Allows much more favorable and flexible use and abuse of campaign donations etc.
Newt/Mitt supporters: “Rick Santorum is a dirty rotten piece of rat feces who should be endorsing my candidate!”
How can we resist such sweet talk?
Rick is still on the ballots and even if he isn't campaigning any votes for him still count towards delegates and he knows that if he is going to primary President Mitt out of office in 2016 he needs to know the final extent of his support now.
He will endorse anybody but Obama when the time comes.