It’s largely historical. France heavily encouraged diesel cars for decades, so there’s now a huge installed base. Once you’ve done that, you can’t suddenly jack up diesel taxes without making a lot of people very angry (and probably setting a few cars on fire).

Yes they tried to increase it a few years ago and failed. Farmers came with tractors to Paris..

We noticed. In the 1980s, it was not uncommon to smell/hear a diesel sedan putting around town. It was invariably something like a Peugeot or a Volkswagen.