Because they built a huge number of reactors
with state subsidy and then stopped?
France is also having trouble with life extension due to cracked reactor vessels, and is unable to build new capacity cheaply - Flamanville is in project management trouble, like Hinkley Point C, and is projected to have higher energy costs than the existing reactors.
And nuclear power is ironically suffering its own climate change effects, it's dependent on cool water: https://www.reuters.com/article/us-france-electricity-heatwa...