It has less to do with the “right-wing” part and more to do with the “establishment” part. In the American political context, populists and other “outsiders” (especially in the South) have a long history of throwing black people under the bus, by doing things like allying with the Klan, or worse (Huey Long ended up allying with Charles Coughlin, who was pretty much a Nazi) for the sake of building enough of a power base to actually fulfil their policy promises. This kind of also applies to women in America, who have a similarly long history of getting burned by “radical” leftist movements (second wave feminism partly started as a reaction to the virulent misogyny of 60s political counterculture).
Given that context, it’s really easy to read an old rich white dude (like Sanders) promising “equality for all” as “I’ll throw black people/women under the bus if I can get equality for white men.”
The fact that you have self-proclaimed leftists like Caitlin Johnstone actually advocating for an alliance between the Sanders Wing and the alt-right (which, tbf isn’t exactly a common opinion) is not helping things.