Could it just be that their time has come and gone? The same thing happened with bingo clubs, once they were thriving, now there are many less but some still do okay. Although folk is experiencing a revival of sorts perhaps those new 'buyers' of the genre are accessing it in different ways (festivals, recorded music) than by going to a club.
It may be that folk needs to be thinking of how to budle itself with the mainstream offer? Presumably folk clubs were the new thing some time ago and tapped into the zeitgeist? T