Wayne Steger helps shed some light on this phenomenon in his new paper "The Parties Decide Among Candidates," which he presented at the State of the Parties conference in Akron earlier this month. As Steger explains,
Whether the political parties unify before or during the caucuses and primaries depends in part on: 1) the relative cohesion of the political party coalitions and 2) the candidates who enter and remain in the race. Nominations involve the interaction of party elites and activists who seek satisfaction of their preferences and priorities, and opportunistic politicians seeking to advance their own ambitions with appeals to constituencies that can form a winning coalition within the party.That is, if a party is highly factionalized, it may have a difficult time unifying in advance of the primaries and caucuses. Relatedly, a party may have a harder time coalescing if there are more high quality candidates involved in the race. As Steger notes, the decisions by Governors Mitch Daniels and Chris Christie not to run in 2012 made it a lot easier for party elites to rally behind Romney. (These two features are not unrelated, of course. Quality candidates may be more likely to enter if a party is more divided, and a unified party may have the effect of forcing quality candidates out of the race, which is arguably what happened to Daniels and Christie.)
Oh, another interesting observation that Steger made in his presentation is that the Republican presidential front-runner three years before the election (think Bush in 2000 or Romney in 2012) tends to run, while the Democratic presidential front-runner tends not to (think Cuomo in 1992 or Gore in 2004). If history is a good guide here, that would suggest Hillary bows out for 2016.