Three main factors.

First, racial: backlash against the civil rights movement, which large portions of white working class people saw as solidly against their economic interest, and certainly against their self-image as 'better' beings.

Second, patriotism: left opposition to the Viet Nam war split left activists from white working class people, who saw such opposition as un-patriotic, and as siding with a national enemy.

Third, 'counter-culture' and 'liberation' issues: many white working class people saw the whole counter-culture movement as an offensive display of indolence and parasitism, and reacted to things like 'women's lib' in the same manner as they did to civil rights for Black people.

The unifying element in all this is that, during the sixties and into the seventies, the left in this country ceased to focus on economic issues, on general betterment of working people, and focused instead on promoting things that many white working people were actively repelled by.