The numbers tell the story -

The Shrinking American Labor Union
FEB. 7, 2015

24.2%: private sector union membership rate, 1973

6.6%: private sector union membership rate, 2014

Source: Barry T. Hirsch and David A. Macpherson

A generation ago labor unions were often a familiar feature of the American workplace, but in private businesses across the country, unions have been shrinking. Today fewer than one in 15 private sector workers belongs to a union, compared with almost one in four back in 1973.

But dwindling union participation in the private sector stands in stark contrast with union membership among public sector workers, which rose sharply in the 1970s and has been relatively steady since 1980 at around 35 percent. Overall union membership has fallen by about a half since 1983, according to the Bureau of Labor Statistics, driven entirely by the decline in the private sector.

The causes of falling union participation are hard to pinpoint but may be attributed to several factors, including the pressures of global trade, technological change, the shift away from domestic manufacturing and a tougher stance against unions from government and corporate leaders ...

more here: http://www.nytimes.com/2015/02/08/business/the-shrinking-american-labor-union.html?mabReward=R4&action=click&contentCollection=Books®ion=Footer&module=Recommendation&src=recg&pgtype=article