Everyone is after cheap fuel prices and diesel prices are no different. There was a time when diesel fuel was extremely cheap in comparison to gasoline, and diesel cars were relatively young and unpopular. As the diesel engine became further improved, diesel became more expensive in relation to demand.
As with any commodity, the higher the running costs, the less profit is available. The more expensive a car is to run, the less likely people are to choose to purchase it. They will therefore look for a more economical vehicle, and the more expensive vehicles will lose revenue. With diesel costs on the rise and more or less parallel with gasoline, people are choosing the cars which get them the best mileage. Even though diesel is excellent for economy (getting almost 600 miles to tank of diesel, compared to 300 of a car), people also consider other factors when buying a vehicle.
Diesel fuel prices have increased over the last few years, and in some areas it is actually more expensive than gasoline. That in itself can affect sales of diesel cars. The effect of that will be a drop in diesel cars sales, which will then affect the diesel fuel sales, resulting in a drop in diesel prices. It is like an ongoing circle of success coupled with failure, similar to the housing market. When supply outweighs demand there is an imbalance in the economical structure, and supply drops to meet the demand. Once demands rise again, the supply is produced to stay in line with demand, and the circle starts over.