Graham is polling approximately zero in Iowa and New Hampshire so it's unsurprising that he'd go. I expect the eventual Republican nominee to be the winner of one or both of those contests but for staying in the race you have to show decently in one or both of those. Graham's hope was that he could stay in the running through those two then dominate the home turf of South Carolina. But with him out that state goes much more up for grabs and Republicans know they need to play strong in the Southern US to have any hope at a Presidential win.
Here's a thing. If you squint your eyes to blur the details, it appears that the sum total of Christie's, Rubio's, and Cruz's poll numbers exceed Trump's. If the three of them could manage to make a deal they could pick one to knock Trump off his current perch. The ideological differences among them are non-trivial so it's not a natural compromise and here's the thing...
Of the three I think Rubio has the best national chances. Christie is just not well known (nor liked) outside the eastern US and the rest of the Republican country has a built-in hostility to eastern intelligentsia who they (somewhat rightly) see as being part of the Washington Insider dirty crew. Watch for the maneuvering later on when Hilary tries to paint herself as not-from-there and everyone else points out she went to New York to run for Senate, not even trying to get a seat down south. But I digress.
If Christie was a less egotistical bastard he'd realize this and throw his support to one of the other two, but see above about differences. I expect him to drop within the next couple months but by the time he does it might be too late. He's most likely to support Rubio (as Cruz's antics have not made him a lot of friends outside the frothing-at-the-mouth crowd) but...
Rubio might go sooner. He's got next-to-nothing going for him in either Iowa or New Hampshire and if he shows as badly as I expect in those two he'll find the money drying up. Then what? Well then it comes down to whether Bush has hung in long enough. If he's still in the race that the party machine and party machine donors have to pick between him and Cruz my money is on Bush. He's insipid and campaigns terribly but he hasn't made a lot of enemies outside his home state. Cruz's negatives loom large and Republican strategists might well decide that he's just too easy a target for the Democrats. I also expect Bush to do well in South Carolina, which is pretty close to home turf for him.
BUT, if the field remains split between Cruz and Bush/whoever that still doesn't top Trump's numbers, unless he causes himself to fall. I used to think that was inevitable; now I'm less sure and based on how the recent Democratic debate went down I'm thinking the Dems may be concluding the same thing.