I think it's more like either
1) the old Devil Fruit users were all killed off. there's only one of each fruit right? So If you want a fruit that someone else has...
2) The New World was pretty much totally unexplored before Gol D Roger showed up. If the DF's all come from the New World (as I suspect) only p[people who were badass without their fruits would have made it as far as the New World in the first place, meaning there are a lot more badass-normals in the old generation