Democrats have a clear frontrunner who is almost certainly going to get the nomination.
Republicans see Trump and Carson as weak frontrunners, so there's more incentive to stick around because of the perception that someone currently in single digits will win.
That's a great point, and one that means basically anyone can be the GOP nominee next year. Certainly, current polling suggests the nominee will be Trump, Carson, Fiorina, or Rubio, and I have my own reasons to believe the nominee will be Kasich or Bush, but if any one of these aforementioned candidates falter, it is possible that even Lindsey Graham or George Pataki--assuming they still in the race until such time--could be the dark horse of the year. It all depends on debate performances, and events leading up to the election and the end of the primary season.
