I’ll answer specifically using examples for the industry I’ve studied, which is coal mining. There are three main factors which seem to lead to union decline.

First, is mechanization. In 1950, the president of the United Mine Workers signed a major contract with the major coal owners. In the agreement, John Lewis allowed mechanization to spread unchecked. From then on, underground miners were consistently replaced either with machinery or with above-ground strip mining (which is very machine heavy, and requires little human labor beyond machine operators).

Second, you having a change in coal company structures/growing corporate hostility. By the late 1970s, global energy conglomerates that have stock especially in oil, begin buying up coal companies. These global corporate owners are much less willing to work with the union, and often break union agreements by closing down union mines and reopening non-union mines. Although the UMW tried to fight back, striking tactics that had prevailed for decades were incapable of forcing the hand of globalized energy companies who were increasingly hostile to union demands. Another big part of growing hostility towards unions was the decline in corporate profit in the 1970s.

Finally, you have the federal government loosening it’s oversight of labor-management relations. With Reagan’s crackdown on PATCO in 1981, and an NLRB (National Labor Relations Board) increasingly indifferent to union issues, the government failed to police the growing hostility of owners towards the unions.

I’d point you to read the works of both Jefferson Cowie (“Stayin’ Alive” and “Capital Moves”) and Lane Windham (“Knockin’ on Labor’s Door”). They have a good debate about union culture, corporate hostility towards unions, and changing politics and culture in the late 1970s and early 1980s. I think you’d find a lot of the things I pointed to would hold true in other major industries at the time, but you would have to ask others about that.