It goes back to the AFl. Gompers led a much more exclusionary union movement, and as time went on became more racist and anti-communist. However the reason a trade union federation was needed and is still a great development for that moment is because the Knights of Labor fell apart and fell apart hard. So a more organized "profession" union of trade workers with the specific purpose of collective bargaining directly with management proved effective in a time of serious reversals of labor power.

The problem is these things are not a substitute for socialism, and in fact create a stronger labor aristocracy. I can't blame the craft unions for taking their shot and gaining those wins for themselves. It is a response that sadly makes sense even if you isolate yourselves. And as organized labor is crushed again, it is obvious that the same process is happening.