It's related to how long ago the uranium was formed:
https://world-nuclear.org/information-library/nuclear-fuel-c...
We can calculate the abundances of U-235 and U-238 at the time the Earth was formed. Knowing further that the production ratio of U-235 to U-238 in a supernova is about 1.65, we can calculate that if all of the uranium now in the solar system were made in a single supernova, this event must have occurred some 6.5 billion years ago.
This 'single stage' is, however, an oversimplification...
The really interesting thing is that phrase "the production ratio of U-235 to U-238 in a supernova is about 1.65"; the now-rare U-235 is actually more abundant than U-238 in the fresh debris of a supernova. Prolonged aging has preserved more U-238 (half life 4.47 billion years) than U-235 (half life 0.704 billion years) to the point that U-238 is now much more terrestrially abundant. If Earth had been formed with uranium that rich in U-235, there would have been Oklo events all over the place. Uranium wouldn't need isotopic enrichment to be used as fuel in light water reactors. Nuclear fission would probably have been discovered early in the 19th century, soon after the element itself was recognized, because any substantial quantity dissolved in aqueous solution would have reached criticality.