Attitudes of parents change with overall social attitudes and expectations. It was not specifically political conservatism that led to the increased conservativism we see now in physical exam settings as compared to years past. In my estimation, the changes come down to four things (NOTE - this is primarily to do with school-based physicals but the larger affect on private clinic settings is certainly related:
1. The end of the draft with now fewer and fewer young doctors having had military service of their own and had in their own personal past experiences a military physical. This one is the weakest of the three, but I think plays a role. It also cannot be overstated just how much attention was paid, particularly as kids went through the junior and senior high school years, on readiness for military service. It was practically a fetish of its own in a societal sense. This was much more reflected in more rigorous standards in phys. ed., but overall student health was part of it, too. The physicals I had for sports in HS were more detailed and comprehensive than the ones I have today as a man in his 50s. ALSO, fewer fathers went through military physicals and no longer had that in their heads as an idea of the sort of things boys needed to expect in life. (This is gender specific, but as everyone on Zity knows, there was a long double standard between boys and girls at the doctor).
2. Rapid changes in technology. This is the broadest of the four but the most unavoidable. They can simply learn more about your body now with less invasive means. In the past, you simply HAD to be undressed for doctors to access the information they needed. That is less and less true now.
3. It cannot be overstated just how sudden and massive an impact on this matter the infamous Franco videos of the 1990s were. He is best known for his college wrestling videos but he also managed to make one of a high school physical at one point. He was sued in absentia for his intrusive videos and the plaintiffs won over $500 million!!!! That ALONE changed the official rules of wrestling at the elementary, middle school, and high school levels to stop doing nude weigh-ins which were, prior to that, generally mandatory (mandatory when I wrestled in the 1980s). The same chill the Franco matter had on school weigh-ins affected school physicals. EDIT: link to article about the suit: https://abcnews.go.com/GMA/story?id=125551&page=1
4. Economics. The rapid growth of employer sponsored health insurance, the rise of penny-pinching HMOs, and the changing financial expectations of medical establishments also plays a key role.