A majority of Republicans oppose Trump as their nominee, and would prefer just about any alternative. To defeat him, however, they would have to rally behind a single opponent, which means that most would have to abandon their preferred candidate. But there is really no rational way to determine which Republicans should have to do this: they're trapped in what game theory calls a volunteer's dilemma. It's a perverse situation, but it's not one that any given voter can do anything about.
For Democrats, the problem is a lot simpler. To guarantee the best chance of beating Trump, the supporters of one of two candidates have to abandon their preferred nominee - but since there are only two options, the calculus is completely straightforward. If you want to beat Trump, your best bet is to nominate the candidate who polls much better against Trump in head-to-head matchups. Constistently, that candidate is Bernie Sanders.
One can, I suppose, maintain that electability arguments against Trump are illegitimate this early in the election. That is not, however, a position that most Clintonites have actually accepted. This places them in a less rational position than Republicans, who, as confused as they are, have no clear solution to the Trump problem. Clintonites do - and they're rejecting it.