Year's of anti union rhetoric is likely to blame. People have been brain washed into believing unions are bad. That they are not there for workers rights but rather in enrich themselves.
My step father works in construction and is very antiunion. He could have much much better healthcare and retirement plan. But he rather trade all that for not having to pay union dues. He is very against unions taking his hard earned money. He works for a family run outfit that's not union. His regular wages are also no where near as good as those people who work in the union, but that is a moot point because he's lucky enough to work mostly on state jobs which are prevailing wage. Everyone gets paid the same rate, and my step father ends up making more than people in unions because he doesn't have to pay dues. My step father used to bitch about Unions not allowing non-union sub-contractors to work on job sites where the main or other contractors are unionized. But that's really not an issue anymore. There are a lot of small state jobs in rural western Massachusetts right now that the larger unionized outfits don't want to touch. The job is either too small or too remote.
Unions are what people need. And I have not doubt the weakening of unions has helped played a role wage stagnation over the last 40 years as well as the rise in cost sharing when it comes to healthcare. Unions could solve this. But people have it ingrained that unions are bad and not the answer.