I think that there are several factors at play here. For example, many naturist clubs tend to be in more remote locations for obvious reasons but the younger crowd want easy and quick access to restaurants and bars when on holiday. Naturist clubs or naturist friendly hotels tend to charge more than 'textile' hotels or resorts and if there is a free beach available then that becomes a more workable option for many. I also think that as a whole, attitudes towards social nudity aren't as relaxed as they used to be and I think that the media is largely to blame for that. We're so indoctrinated about what the perfect body should look like, body shaming is on the rise, pharmaceutical companies are forever telling us what to buy to look better as well as the fashion industry. Hardly anyone is advertising body acceptance and tolerance. Add all of these ingredients together, along with a few more that I haven't considered and purposely going somewhere to get your kit off becomes less appealing.
Maybe it will be a peak and a trough thing. The 60's were very liberal and today not so much. So on the basis of that swing, maybe we'll see a change in attitudes in the next 50 years...