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