My parent's generation in the UK experienced things like the three day week ( https://en.m.wikipedia.org/wiki/Three-Day_Week ), major disruptions to normal life due to strikes. In the end, the Conservatives in some ways 'won' and so this has gone down in the cultural memory as unions having 'too much power'. Add to this that, largely due to unrelated geopolitical and macroeconomic reasons, the 70s (when unions had more power) were hard times, and in the 80s (when Thatcher and Reagan were doing their thing) things felt like they were getting better for lots of people (even though with hindsight inequality was starting to grow and the seeds of many of today's problems were being planted).

Maybe someone else would be a better judge on what the source is. I know the UK had a period of more entrenched socialist policies prior to Thatcher that may affect the general population's perceptions of the movement. The poisonous Murdoch newspaper/media ecosystem can't help either.