the NLRA’S impact in the workplace

The National Labor Relations Act was passed in 1935. In 1944, 35% of the workforce were members of an organized union. In 2016, private-sector workers (those workers not in the government, schools, etc) only had 6.4% participation in unions.

What do you think are the reasons?
Has the culture changed?
Is the National Labor Relations Act as important now as it was?
Are the reasons for unionization not as valid and they were?
If that is the case, why are 34.4% of public sector employees unionized today?