French scientists were at the forefront of scientific discovery throughout the 19th Century and into very early 20th Century. They were leaders in Mathematics, Biology, Chemistry and atomic Physics. And then something happened.
The most common explanation(s) are the two world wars.
A competing narrative is that the giants of 1900-1910 studied radiation and three of them (Curie, Becquerel, and Moissan) died in their early-50s due, in part, to naivete with regard to safety requirements. That is, they were killed by their research.