Why does the Federal Reserve seem to hold so much weight on the unemployment rate, relative to labor participation? The title is the question. 

In the press conference held yesterday, the Chair referenced "unemployment" 18 times, relative to "participation" 3 times.  If the labor participation is at the lowest since the 70's, is it delusional to buy into the seemingly marketed strength of the U.S. economy?
As mentioned in the press conference, other data is taken into consideration, such as the Employment - Population Ratio, especially when filtered/line series graphed to detail trends in demographics.

This blog post provides a bit more insight on the data. 

It seems relatively, the U.S. is climbing to employment-pop levels we haven't seen since the mid 80's, which seems to suggest the outlook isn't as terrible as initially thought.