[maximente]

without writing a thesis, here are some thoughts:

- US is/was about 100 years behind european labor movement (germany, sweden, etc. started mid 19th century; US not until 1935 Wagner Act)

- ghent system - in scandanavian countries, unions provide most unemployment benefits, not .gov. you can imagine this leads to large union membership

- americans lost manufacturing/blue collar ("union" jobs) en masse as white collar jobs rose, whereas trade skills are still a thing in countries like Germany with their sort of parallel university system for trades

- there is probably a relationship between strong social welfare and union membership, though directionality isn't clear - perhaps large % of union membership enables more tolerance towards social welfare, or perhaps social welfare allows people to not worry about retaliation from attempting unionization, or ..