That's how it looks on the surface only because the alt-right never trained their sights on Sanders, Warren, and the like. Indeed the progressive/socialist wing was seen by the alt-right as a useful tool to further weaken Clinton for the general election; she could not tack toward the middle after the convention, lest the Bernie bros rise up and smite her some more. So the attacks from the right on the progressive candidates (I include Hillary in this group for this purpose) were not symmetric or evenly distributed, but all trained upon one candidate. Had Bernie somehow shocked the world and upset Hillary for the nomination, the alt-right would have quickly found ways to chip away at Bernie's likeability. In the minds of middle America he's just as elitist as Hillary; he comes across as a rumpled college professor, which the far left loves and the vast middle detests. And this is apart from the main message the Trump side would have adopted, that the election was now a referendum on free enterprise versus socialism. The character assassination of Hillary Clinton was achieved over many years, so that by the end the idea of anyone even having a "private server" was somehow equivalent to treason. But the main concepts of doing so would have carried over just fine if a different candidate had emerged. As an aside, my recollection is that Trump rarely used the term "rigged" until Clinton had the nomination locked up. It was useful to let Sanders deliver the body blows for as long as possible. The most Trump ever had to say about Sanders was "crazy", in an amused tone. After a while, he started to add, "but he's right about one thing..."