Several factors but primarily because the Baby Boomers are retiring at an incredible clip and the millenials, in a lot of cases, are not qualified or trained to fill those jobs. http://www.factcheck.org/2015/03/declining-labor-participation-rates/
Do me a favor. When you hear a stat that sounds either really good or really bad, just do a bit of research. This took me 15 seconds to find... http://www.tradingeconomics.com/united-states/labor-force-participation-rate Click on the max button and you will see that we are actually right at the historical average. Also, @Tiger in NC is correct when he says we are in the middle of a demographic shift and that's the reason we're lower than we were 20 years ago. This is not news to anyone. We've been talking about baby boomers retiring for years.
