Unions more or less kinda worked when you had largish enterprises with a lot of employees. This is more or less the world described by John Kenneth Galbraith's "The New Industrial State."
This world doesn't really exist any more. It was undermined by lots of forces. I'd say the main force was people being dissatisfied with the working conditions.
In order for collective bargaining to work, there kind of has to be a collective to bargain for. Humans are not naturally attracted to collectives.