Unionized companies were essentially a conspiracy between the unions and the companies to fleece the consumer. As long as the consumer did not have an alternative to the artificially high prices necessary to support profitable business with union wages, the conspiracy could go on.
Once consumers had an alternative, due to deregulation of airlines, importing of cars, non-union firms entering the field, etc.; the artificially high prices could not be sustained and the foundation necessary to support the high wages and benefits of the union workers disappeared.
IOW, union workers did quite well for several decades by shifting the costs to everybody else. In the process they reduced everybody else's standard of living by at least as much as they increased their own.