So I'm in the middle of the millennial age group (born in 1990) and I'd say a lot of what has been said here is accurate. My view on it is that with the rise of snapchat, facebook, etc, there is an increased need to "show off" as everyday you're flooded with pictures of things that your friends are doing. This leads to many wanting to do as many different, cool things as they can as I think this helps one think they are more "well-rounded," which is important to people my age. Especially since a lot of dating these days is online and seeming "interesting" at first glance is quite important.
On the social side, people just talk to each other less in general and so I think any opportunity to meet new people and have an excuse to talk is attractive.
Triathlon on the other hand is a bit of the opposite that requires many long hours of training, maybe by yourself, to get a reward. It really requires someone who gets pleasure from self-improvement just for its own sake, which is rarer these days.
Personally speaking, I happen to have a self-improvement personality and spend my time training, trying to learn languages that I find interesting (Japanese and German - I like efficiency I guess), keeping things in order (chores, etc), and trying to figure out what to write next on the blog (
https://www.engineeringfitness.org - I appreciate the visit!). And most of the day is working towards the PhD, which unlike ungrad work, never seems to have an end point and drains an ungodly amount of time, sometimes.
I like analyzing things -
http://engineeringfitness.org