2B years ago, the U235/U238 ratios were different. Isn't that (part of) the explanation?

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto... says:

> A key factor that made the reaction possible was that [back then] the fissile isotope ²³⁵U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors.