From the NBER:
“Stagnant wages and a declining share of labor income in GDP in recent decades have spawned a number of explanations. These include outsourcing, foreign competition, automation, and the decline of unions.
Two new studies focus on another factor that may have affected the relative bargaining position of workers and firms: employer domination of local job markets. One shows that wage growth slowed as industrial consolidation increased over the past 40 years; the other shows that in many job markets across the country there is little competition for workers in specific job categories.”