Maybe because the Boomers grew jaded with it as they grew older and passed that attitude on to their children the Millenials?

More loyal than whom?

More loyal than Boomers? I doubt it.
More loyal than Millennials? Probably.

But that's what you'd expect to see, isn't it, if society at large was making a slow transition away from religion? The oldest apostates will be few, and numbers will grow in subsequent generations.

Quite likely, that was the the time of a big down turn in educational standards, before the modern reforms began to kick in.