In many ways, there was no reason to have movie ratings. The studios agreed to the Hayes code, which uphold certain standards about language, nudity and gruesome violence. Arbitrary, ridiculous standards, but generally understood by the audience.
By the late 1960s, all this fell by the wayside as film, like other popular media, responded to an opening up of the culture and a liberation from repressive societal standards (and instituted some new ones, but that’s another sixty or seventy columns). Filmmakers wanted to show how people really talked and really looked and really acted.
Hence, a rating system. It wasn’t great.