Good point. You can't make good candidates step up, but you can certainly work hard to make unwanted candidates fail. I think both of you are right about the national committees. It's unfair of me to lay blame for candidate choices or success at their feet. Regarding the DNC, I recall something in the hacked emails that suggested they favored Hilary Clinton over Bernie Sanders, and acted on that preference. But that's not the same as wanting someone over Clinton who, for lack of will, didn't run. Once the voters had selected Clinton as the candidate, they were stuck with her and all the baggage she brought with her.
I think I'm soured over Utah politics and extrapolating that irrationally to the national level. Briefly, the Utah GOP used to have a caucus/convention-only system for choosing candidates for the primary ballot. When the Tea Party basically took over the state party leadership, they lobbied for far-right delegates to the convention. Since then, the convention voting has skewed quite a bit farther to the right than the general GOP voting in the state. Many Utah Republicans are surprisingly moderates, but they were given only arch-conservatives (cough, Mike Lee, cough) as credible candidates in the primaries. Because of the circumstances of districting, it has become difficult to unseat these unrepresentative delegates. Displeasure over this led to various initiatives resulting in, among other things, S.B. 54, a law that allowed candidates to qualify for any state-run primary by collecting signatures. The Utah GOP literally bankrupted itself fighting this in court, losing finally when the Supreme Court denied certiorari for an appeal from the 10th Circuit. Rank-and-file Republicans saw the law as one of only a few ways they could get popular moderate candidates like Mitt Romney on the GOP primary ballots. (Romney came in second at the GOP convention but won the GOP primary in a landslide.) Another result was the United Utah party, composed mostly of disaffected Republicans and a few moderate Democrats.
Sorry to bring up local politics so much. It's where my understanding of politics comes from. I realize this is a national-politics thread, centered on Donald Trump.