A very interesting article in the WSJ. Check it out here.
I noticed this back in 2013 or so at a half marathon when the announcer at the start said that 60% of the registrants were women.
Looks like the popularity of running has peaked at around 2012. I started running around 2007....it's about double what it was then!
Back in 2010 I noted that number of searches for the term "half marathon" was growing strongly each year. Using google trends again for this search we can see that indeed 2012 seems about the peak and especially after 2013 it starts going down.
It's going down pretty steeply...I can't help but note that 2013 was the Boston bombing year.
If that is the cause, my guess is that people are still running the same amount but maybe they are racing a wee bit less because of the bombings.