The fallacy of self-interest
The Republican Party's Trump problem is, at its root, an issue of collective action – each candidate considers it in their interest to continue campaigning, thereby preventing anti-Trump voters from coalescing behind one alternative. Ted Cruz is this figurehead behind which the GOP should rally – the more moderate and more electable John Kasich holds too few delegates and as such, barring any drastic convention rule changes, he has, mathematically, no chance on the first ballot. Although Cruz's fate is now clear, prolonged campaigns by candidates who thought they would prevail despite a depressing primary scoreboard and cratering poll numbers (*cough* Marco Rubio, *cough* Jeb Bush) prevented Cruz's emergence from the pack and eroded any restraints on the Trump juggernaut.
It's interesting that this problem of competing individual aims should play out in the GOP primary, a party which espouses man's autonomy and finds its ideology founded upon the power of individuals acting in their self-interest. The 2016 GOP primary has exposed the weakness of those views, particularly outside the economic realm.
Even if the party establishment did manage to orchestrate an anti-Trump crusade, The Donald's white working-class base has been seething for some time. It is doubtful that Republican elites could have completely prevented his success in the face of such socially cohesive resentment. Further, any corralling of Trump supporters into the party's doctrinal fold would only postpone the inevitable separation of base and establishment. However, one does wonder whether, had a Trump arisen on the left, the Democrats would have proved more able to kill Trump's surge in its cradle.
Credit to Sam Seder of The Majority Report for enunciating this specific aspect of 2016's Republican delirium.