Hays had much more rigid standards explicitly laid out, such as an outright ban on all sexual content, and didn't allow much leniency even for content intended for adults. The MPAA rating system is rather arbitrary and allows almost anything for adult content, which inevitably enables the line of what is adult only material to move towards kids. While the Hays code would almost certainly have been subverted as well, it would have taken much longer and been more difficult.