In my experience, the clubs cater to a niche. They’re effectively a kink in and of themselves and there’s a very explicit innuendo that sex is to be had on premise. Nude beaches on the other hand are a casual exploration of being clotheless. And while sexuality plays a part, it is not the focus of the experience and is often generally discouraged. I think nude beaches are cultural and thus more prone to the ebb and flow of said culture. Swingers clubs are a fetish and will exist regardless of culture as long as there are people with the kink.