Guessing here, but nuclear power is heavily regulated in most places, and maybe there are some French rules about what to do when there is excess production.
Would be interesting to know a real answer, though.
Discussion
Yes it's probably because the nuclear plants are run by the government, and they need to approve it. That's a lot of nuclear though... Close to 80% of their energy production if I remember correctly. Mining bitcoin with the excess would be the right choice. A great signal to Europe too.