Union power began to diminish over the years and by the 1970's with the growth of corporate power and influence and control of the government weakening worker protections and labor laws to such an extent that organizing began to suffer extensively.
This together with the enormous shift from an industrial and manufacturing society to a service white collar based economy hurt labor membership. White collar workers were easily frighten about potential conflict with their employer and most employers would not hesitate to use tactics to intimidated employees who may consider starting a union. Especially with government refusing to enforce the law that was meant to protect their organizing.
Organized labor's own internal corruption problems and inability to modernize it's out of date message also added to the problem of attracting new generations of members.
"Labor is prior to, and independent of, capital. Capital is only the fruit of labor, and could never have existed if labor had not first existed. Labor is the superior of capital, and deserves much the higher consideration." Abraham LincolnWashington State has always been among the states with the highest number of workers per capita who were members of unions. It was more unusual for a worker not to be in a union then to be unrepresent not that many years ago.