I believe the popularity of diesel car in Europe is actually a tax-related hack.

The idea is that diesel is the "work" fuel, for shipping, construction, etc... While gasoline is the "consumer" fuel, for personal use, motorsports, etc... Make the former expensive and it will affect the entire economy, everything will become more expensive and less competitive. Making gasoline more expensive will not have the same impact.

So, put high taxes on gasoline. The result was an increase in popularity of diesel cars, that cost less to run because of taxes.

Now, the situation is changing. Diesel, at least the one that is legal to use on the road is taxed at a level closer to gasoline. Diesel cars are also becoming less and less welcome with regards to low emission zones and green taxes, so many people are going back to gasoline.

Yes, in France as I pointed out in my other comment, the diesel fuel was always cheaper than regular gasoline. The re-alignment of the tax was (amongst other things) what sparked the massive yellow vest protests in France in 2018.