Hard to say, really. In my team I think the stated reasons are most often "I saw it at a folk festival and it looked like fun, so I gave it a go" or "my friend saw it at a folk festival and thought it looked like fun, so they gave it a go, and then they made me give it a go". In my case, it was because I was quite into folk music first, and then when it became necessary to actually start getting some exercise, folk dancing seemed to dovetail quite nicely.

I think part of it is that to some extent anything that has a reputation for being a bit naff tends to draw in people of a countercultural bent. And part of it ties into the fact that folk in general seems to be having a bit of a resurgence. I suppose another part of it might be what I hear the kinds these days are calling whimsy-maxxing.

I can't speak much to why folk in general is having a resurgence, but in my case it's because of complicated feelings about Englishness. A lot of English people these days seem very concerned about patriotically "preserving their culture" and as far as I can tell what they mean by that is drinking in pubs and football hooliganism. Either that, or harkening back to the glory days of empire when men had stiff collars and stiff upper lips, and women baked cakes and knew their place. In contrast to Scotland and Ireland, where their traditional music and dance is much more visible, England seems to be actively embarrassed by anything that's uniquely English that you wouldn't find in an Agatha Christie novel. I, and I think others, want a way to connect to my heritage that is kind of in opposition to that kind of conservatism, if you get what I mean?

(No judgement intended on people who are into football, ale, or vintage stuff in a healthy way, though.)