I stopped going regularly after a little over 5 years, mostly because I was plateauing in my dancing and getting bored with it. In my experience, you plateau multiple times, just as with any skill, but you find a way to get better and more creative, but at that point, my heart wasn't into improving anymore so I wasn't enjoying it as much. I definitely keep telling myself to get back into it, however, because it's such a great social outlet.
I'll also add that it's a really great activity because it's physical, it's creative, and it's social. Partner dancing forces you to learn how to work with other people and communicate better. Not everyone you dance with has the same skill level and you might think you're doing well communicating (when you're leading) but after dancing with several people you find out right away where you're not doing a good job.
I also did find that it made me more aware of my movements and improved my musicality. I got better at watching other people dance and seeing what movements they made and how they choreographed their "routines". That kind of tickled a different part of my brain I hadn't used before in any other activity. The "plateauing" I mentioned earlier forced me to get better at watching others and repeating what they did as well as seeing what I was doing and trying to change it up if it was getting stale.
Many of my friends (late 20's, early 30s) wouldn't dream of being the first on a dance floor, or trying a class. I think most would rather shoot hoops or go hiking. Dance doesn't even occur to them as an option.
My personal theory is that there's a stigma. Social dancing is what old people do. (Although I don't want to discount the k-pop dance scene I witnessed in college. That had a more performative aspect to it though, like marching band).
(based on your username), Poland has a great scene by the way.