Why would the United States Bureau of Labor Statistics show work force participation has been in a steady decline for 6 years?

Because that means that more people are on the government dole which more or less negates any apparent increase in employment rates. It's equivalent to robbing Peter to pay Paul.

Because fewer people are looking for jobs. At this point ten thousand people a day are retiring. The leading age of baby boomers is sixty seven. As time goes on look for that number to increase. Two other reasons are that some people decided to work for themselves, and some went back to school to sharpen their skills. And a normal number decided to stay home and have babies now that the worst of the recession is over.

There are 92.6 million people that have stopped looking for work.