The labor force participation rate is the lowest that it has been since 1978. The graphic below shows how it has fallen since 2003.

Brad Plumer at the Washington Post offers three good reasons for this decline:

1. America is aging; baby-boomers are now retiring.  Though this is certainly true, it, alone, is not enough to explain the entire decline.
2. The bad economy is keeping workers in school and out of the labor force.  This story is bolstered by data that shows that the rate entry to the labor force is declining.  We know more young people are choosing college and this makes a difference.
3. More workers are going on disability insurance.  According to Plumer, there are now twice as many workers on disability insurance (8.8 million) than there were in 1995.

If you are interested in this topic, you should read Plumer's post - he presents both data graphics and research to support his arguments.