It doesn't matter how insignificant the modifications are. As long as there are any at all, most stations won't lift a finger until there is an actual (as in not hypothetical) consumer demand for it. Which creates a chicken-and-egg problem because not many people will buy the car (pay the extra price for the new engine vs a proven old petrol/diesel engine) until the filling station coverage is at least 90%. Of course, with political pressure things could happen faster but the core business of car makers like Citroën/Peugeot is to sell cars to people. They do that by designing cars according to the wishes of the majority of those wealthy enough to buy brand new cars.