The book Bowling Alone by Robert Putnam helps understand the reasons
A lot in there to digest, but one point to raise/highlight is that interest or otherwise in a subject is not defined by the number or population of clubs devoted to the subject - obviously naturism, as that’s our topic, but most other things too. Certainly in the UK the membership of formal and organised groups has fallen, but that doesn’t mean interest or participation has followed it.
Anecdotally I’d put this down to people having more choice of things to do and not wanting to commit to one particular thing to the degree required by club membership; following club rules can be another deterrent. And let’s be honest, club organisers (whatever the subject) can get quite demanding over the rule book! Just doing your own thing in your own way in your own time is more attractive.
As an example, we’re told that since covid, lockdowns and home-working more people have dispensed with clothes more of the time. But of course that same environment and social distancing means people are less likely to explore naked living via the club route.
This is good stuff thanks for making the effort to research and put this together