This is mostly due to the fact that in the 1960's, the New Left, unlike it's classical predecessors, went from championing the causes of workers and farmers to the causes of racial "minorities," feminists, and homosexuals. The main reason for this, of course, is that Marxists couldn't make many inroads into the American labor movement. Regardless of what their economic views were, you'd be hard-pressed to find a Teamster who was a fan of the Viet Cong. So the New Left decided to go to greener pastures of people receptive to their message (radicalized racial groups) and abandon the white working classes.
True, though they still maddeningly insist on claiming to be for "the working class" or "ordinary people"--even if they are Ivy League professors who think ordinary people are stupid.
Actually, not every left wing group abandoned the white working class (though the ones that didn't have no influence). Bob Avakian was horrified by the contempt the wealthy college-educated whites of the New Left had for the white working class and told them their class roots were showing. They replied this his racial roots were showing (Avakian is, of course, Armenian). Today the Progressive Labor Party (originally Maoist and then "Hoxhaist") opposes all nationalisms, including those of the "oppressed," and laments that the driving force of revolution in the world today is nationalism (which it is). Another leftist organization called Ray-O-Light actually advocates national self-determination for poor whites just as they do for Blacks and Hispanics. But as I said, these groups have no influence.
Unfortunately, long before the New Left many advocates of poor whites were excoriating Blacks as "the enemy." Perhaps if they had not done this things would be different today. We'll never know.
Unfortunately, the continual advocacy of Blacks by wealthy whites has led to poor whites hating Blacks more than ever. Just as their ancestors associated slaves with wealthy aristocrats now they regard Blacks as the favored pets of the wealthy whites and resent them all the more for it. It is simply not possible for poor white people to get a hearing in a country obsessed with genes and chromosomes and where the poorest and most unfortunate white person is categorized as an "oppressor" who must kowtow to wealthy Black university professors. Ironically, it was none other than Karl Marx himself who pointed out that wealthy British matrons seemed to care more for far away population groups than they did for poor English factory workers. I guess that makes him a "regressive, right wing populist" by today's standards.