According to Melanie Mitchell's book, Complexity: A Guided Tour [PDF], Mendel's model
[...] was published in a rather obscure journal and was not appreciated as being of great importance until 1900, after which several scientists had obtained similar results in experiments.
The Wikipedia page for his paper describes his work as "being largely forgotten". So, how was his work rediscovered again, given that 45 years are very long comparing to human lifetime?
I hope that I am not exaggerating, but either someone had to read his paper and still remember it after 45 years (which contradicts the claim of its "being largely forgotten"), or re-read every old paper just to come across his (which has a very low chance with an obscure journal).