The labor force hasn’t grown because the economy hasn’t. It really is just that simple. Because the economy isn’t actually growing, the labor market has been for a very long time caught in a chronic state of feebleness and malfunction. In linear terms, there are only positive numbers but that doesn’t matter nearly as much as the intensity of them.
Without actual growth, far, far too many Americans remain on the sidelines of the jobs market. Worse, this weakness is reinforced forward as job growth isn’t nearly enough to absorb possible new entrants, either (population growth). That’s the participation problem being derived directly from continued cyclical underperformance.