Factors that you feel have led to the decline in union membership over past few decades

Summary of the top 3 factors that you feel have led to the decline in union membership over past few decades.  For your “top 3” comment on any ties to the concept of “organizational justice” . what are your thoughts in terms of strategies that unions could utilize to gain back importance in the U.S. workforce and are there any international concepts, practices, etc. that U.S. unions should consider?