Short answer: A shortage of grownups.
I have said, and Jennifer Rubin has said, and who knows how many others have said, that there was simply no reason and no excuse for allowing the situation to develop that led to Trump’s being the Republican nominee. What a piece of great political theater it would have been for Reince Priebus and Paul Ryan to lure the 16 Not Trumps into an undisclosed location, lock the doors, and say that the group wouldn’t be released until they pulled up their socks, acted like grownups, and got behind one candidate who could defeat Trump. Wouldn’t that have been great? But probably not too legal. Okay. How about a few phone calls? “Hey Ben, this is Reince. Right, that Reince. How many Reinces do you know? Anyway, I was just calling to ask if you could see your way clear to dropping out of the primaries. Just asking. We sort of need to not have that Donald guy as the nominee. You’d be a hero. We’d let you give a speech at the RNC. You will? You’ll make the announcement tomorrow? Great! Listen, gotta go. I have fifteen more calls to make. See ya!”
But I’ll let National Review make the case:
“How the Overstuffed Primary Field Led to Trump’s Victory”
And a little bonus article, from some internet browsing I did this afternoon, originally written back when it was very unpopular among conservatives to say that Clinton was better than Trump:
“Don’t Love Hillary? Fine, but vote for her anyway.”