Here's a possible factor in the decline of communal showering you probably haven't considered:

The fall of the Berlin Wall and the collapse of the Soviet Union.

No, I'm quite serious. A lot of men's fashions during the Cold War were linked to a military aesthetic. The super-short buzzcuts popular for men and boys during the '50s were quite obviously linked to military haircuts, for example.

I think there was a constant awareness and subliminal fear that WWIII could break out at any time, and that young men would have to be inducted into military service if such a conflict arose. Therefore, it would make sense to get them used to shower conditions that they'd balk at otherwise.

Once the Wall came down, and the constant fear went away, I don't think there was a continued perceived need to force young men to shower communally whether they liked it or not.