[aecay]

My guess (for men, at least), is the still only partial eradication of homophobia from society. Before the 1970s or so, it was not threatening to be naked around other men because homosexual advances could be violently punished with relatively little fear of consequences. (This is at least my understanding, I was not alive at the time.) In present day European culture (including North America, Australia, etc.) expression of homophobia is not generally acceptable, but enough straight people are privately homophobic that they are not willing to participate in semi-public nudity. The exceptions to the generalization that group male nudity is not acceptable are either subcultures where overt homophobia is still practiced (fraternity initiations, some sports teams), or unusually "woke" spaces/groups.

In principle, we eventually may reach a stage where homosexuality is so destigmatized that straight men are not bothered by the possibility of being checked out while naked, and that might make public nudity once again acceptable in all-male spaces. I don't think it is particularly likely to happen soon given that there are other simmering cultural disagreements about how bodies relate to sexuality (e.g. how genital anatomy and gender identity interact) -- I think that consensus and stability around those questions are a precondition for the acceptability of same-sex group nudity.

The considerations for fully public (and by necessary implication all-gender) group nudity are broader, but I think an element of the above analysis is in the mix there too.