That's a good question! I remember when I was a kid a lot of people would let their cats outside. I have 5, only 2 of whom go outside. One we adopted as an adult, and he was already an outdoor cat, so it wouldn't have been fair to him to change that. The other one we adopted as a kitten and intended on keeping him as an indoor cat, but he decided one day that he wanted to see what the outside world was all about, as he decided that he liked it. We tried to discourage him from going outside at first, but then gave in. There is no humanly possible way to tell a cat it can't do something Our other 3, also adopted as kittens have shown no interest in going outside, so we keep them inside.
I think the difference between then and now is people just don't feel as safe as they used to.