I think it's more like why FEWER millennials are joining...and the answer could be right in front of our noses.
Outside of the obvious trends in participation over the last 10 years, I think an easier answer is found in (sad) county club demographics.
Private clubs are - for the most part - disproportionately white and Republican. I can't back that up with any published statistics, but...duh...everyone knows it...;-)
Millennials, on the other hand, are increasingly democratic and the exclusivity of private clubs (and golf) is perhaps no longer appealing.
https://www.people-press.org/2018/03/20/1-trends-in-party-affiliation-among-demographic-group[size=78%]s/[/size]You would think that basic population growth in the US would fuel club growth. (Heck, it's fueled growth in virtually every other possible thing!) But, as the population has risen from 225M to 330M people as I have been an "adult". the participation at private clubs has dropped as per the article DT cites.
OK, for those who seek to fact-check me, let's just start in 2000 when US population was 280M.
US population up 20%.
US Country club participation down 20% in same period.
Millenials who identify as Republicans down by almost 35%.
Just sayin'...;-)