Like others here said, it's a combination of things. Attitudes change over time.
1. In 1994, the ACLU threatened to sue a school district on behalf of a student who was unhappy with the mandatory showering policy. Rather than getting into expensive lawsuits that they might lose anyway, school districts just decided to drop the policy. The ACLU said the policy "violates students' privacy by forcing them to go naked in front of their classmates."
2. Increased awareness of gay/bi guys sharing locker rooms with them. In the 90s and earlier, most gay guys were in the closet, and some guys are uncomfortable at the idea of being checked out naked by other guys.
3. Smartphones, everyone has a video camera in their pocket now, and there are a lot of secretly recorded locker room spy videos out there.
Enough stories like this, and it's easy to see it from the parents' point of view:
Former Jackson coach gets 15 years in prison for recording boys in shower
Yes, non-athletes who didn't grow up regularly using locker rooms/showers.
As you can see from the videos I linked above, most athletes don't have that issue at all, even today.
I graduated from college only a few years ago, and it wasn't an issue for our sports teams at all.