We need to consider the hobbies that are in vogue. Karate was a big deal when I was growing up, but now, you're likely to see only kids practising as one of those "let's keep the little buggers busy" activities. Hey, building model kits was mainstream (or so we thought) in the sixties, and now the vast majority of builders are middle-aged or old guys.
Here's a graph describing hobbies and activities in the US: cooking and baking is #1.
When it comes to the most popular hobbies in the United States, cooking / baking tops the list.
www.statista.com
"Sports and fitness" is near the bottom, and one doesn't have to think too hard to imagine that martial arts is probably well behind working out, basketball, softball, and other mainstream sports.
Karate just isn't
popular any more, and that has nothing to do with the art. The older folks are probably acting on a desire to get involved when they were younger.