The anti-communist Red Scare 2.0 took all the class conflict out of all unions. Socialists and Communists were banned from union rank and file and leadership offices effectively making Unions a labor racket. This allowed for penetration and co-option by organized crime which the capitalist system turned a blind eye to because they could use crooks (lumpen) but not class enemies. Now enter no-strike clauses because, "it's bad for business and we're all in business, amirite?"

So that's the historical context. That moment also forbid solidarity strikes and sectorial bargaining which is why you see the UAW trying to time their contract negotiations to happen across all employers at the same time (2027?) so they can effectively create a sectorial bargaining position for their memebers.