The Chicago Fed attributes part of the decline in US labor force participation to the baby boomer phenomenon producing a growing number of retirees, but this chart from their newsletter excludes retirees and highlights the real problem.
Female LFPR are expected to fluctuate by about 1 percent (from 1987 to 2020) while male college graduates have fallen by about 2 percent. Male high school graduates, however, have fallen by 6 percent and do not look like recovering any time soon. The primary cause is the declining manufacturing sector and loss of construction, banking and real estate jobs as a result of the housing market crash.